Lithographic apparatus and device manufacturing method involving a heater

ABSTRACT

A lithographic apparatus is described having a liquid supply system configured to at least partly fill a space between a projection system of the lithographic apparatus and a substrate with liquid, a barrier member arranged to substantially contain the liquid within the space, and a heater.

This application is a continuation of co-pending U.S. patent applicationSer. No. 16/376,535, filed Apr. 5, 2019, now allowed, which is acontinuation of U.S. patent application Ser. No. 14/882,241, filed Oct.13, 2015, now U.S. Pat. No. 10,254,663, which is a continuation of U.S.patent application Ser. No. 13/288,831, filed Nov. 3, 2011, now U.S.Pat. No. 9,188,880, which is a continuation of U.S. patent applicationSer. No. 12/869,560, filed Aug. 26, 2010, now U.S. Pat. No. 9,268,242,which is a continuation of U.S. patent application Ser. No. 11/205,325,filed Aug. 17, 2005, now U.S. Pat. No. 7,804,575, which is acontinuation-in-part of U.S. patent application Ser. No. 10/917,535,filed Aug. 13, 2004, now U.S. Pat. No. 7,304,715, the entire contents ofeach foregoing application is hereby incorporated by reference.

FIELD

The present invention relates to a lithographic apparatus and a methodfor manufacturing a device.

BACKGROUND

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.comprising part of, one, or several dies) on a substrate (e.g. a siliconwafer). Transfer of the pattern is typically via imaging onto a layer ofradiation-sensitive material (resist) provided on the substrate. Ingeneral, a single substrate will contain a network of adjacent targetportions that are successively patterned. Known lithographic apparatusinclude so-called steppers, in which each target portion is irradiatedby exposing an entire pattern onto the target portion at one time, andso-called scanners, in which each target portion is irradiated byscanning the pattern through a radiation beam in a given direction (the“scanning”-direction) while synchronously scanning the substrateparallel or anti-parallel to this direction. It is also possible totransfer the pattern from the patterning device to the substrate byimprinting the pattern onto the substrate.

It has been proposed to immerse the substrate in the lithographicprojection apparatus in a liquid having a relatively high refractiveindex, e.g. water, so as to fill a space between the projection systemand the substrate. The point of this is to enable imaging of smallerfeatures since the exposure radiation will have a shorter wavelength inthe liquid. (The effect of the liquid may also be regarded as increasingthe effective NA of the system and also increasing the depth of focus.)Other immersion liquids have been proposed, including water with solidparticles (e.g. quartz) suspended therein.

However, submersing the substrate or substrate and substrate table in abath of liquid (see, for example, U.S. Pat. No. 4,509,852, herebyincorporated in its entirety by reference) means that there is a largebody of liquid that must be accelerated during a scanning exposure. Thisrequires additional or more powerful motors and turbulence in the liquidmay lead to undesirable and unpredictable effects.

One of the solutions proposed is for a liquid supply system to provideliquid on only a localized area of the substrate and in between theprojection system and the substrate using a liquid confinement system(the substrate generally has a larger surface area than the finalelement of the projection system). One way which has been proposed toarrange for this is disclosed in PCT patent application WO 99/49504,hereby incorporated in its entirety by reference. As illustrated inFIGS. 2 and 3, liquid is supplied by at least one inlet IN onto thesubstrate, preferably along the direction of movement of the substraterelative to the final element, and is removed by at least one outlet OUTafter having passed under the projection system. That is, as thesubstrate is scanned beneath the element in a −X direction, liquid issupplied at the +X side of the element and taken up at the −X side. FIG.2 shows the arrangement schematically in which liquid is supplied viainlet IN and is taken up on the other side of the element by outlet OUTwhich is connected to a low pressure source. In the illustration of FIG.2, the liquid is supplied along the direction of movement of thesubstrate relative to the final element, though this does not need to bethe case. Various orientations and numbers of in- and out-letspositioned around the final element are possible. One example isillustrated in FIG. 3 in which four sets of an inlet with an outlet oneither side are provided in a regular pattern around the final element.

In European patent application publication no. EP 1420300 and UnitedStates patent application publication no. US 2004-0136494, each herebyincorporated in their entirety by reference, the idea of a twin or dualstage immersion lithography apparatus is disclosed. Such an apparatus isprovided with two tables for supporting a substrate. Levelingmeasurements are carried out with a table at a first position, withoutimmersion liquid, and exposure is carried out with a table at a secondposition, where immersion liquid is present. Alternatively, theapparatus has only one table.

Although providing improved resolution, the introduction of an immersionliquid may cause errors in the image generated on the substrate,including alignment errors between one layer and the next (i.e. overlayerrors), defocus and aberrations.

SUMMARY

It is desirable to provide a system that reduces lithography errorsarising from the immersion liquid.

According to an aspect of the invention, there is provided alithographic apparatus, comprising: a substrate table constructed tohold a substrate; a projection system configured to project a patternedradiation beam onto a target portion of the substrate; a liquid supplysystem configured to at least partly fill a space between the projectionsystem and the substrate with liquid; a barrier member arranged tosubstantially contain the liquid within the space between the projectionsystem and the substrate; and a liquid evaporation controller arrangedto control a net rate of evaporation of liquid supplied by the liquidsupply system.

According to an aspect of the invention, there is provided alithographic apparatus, comprising: a substrate table constructed tohold a substrate; a projection system configured to project a patternedradiation beam onto a target portion of the substrate; a liquid supplysystem configured to at least partly fill a space between the projectionsystem and the substrate with liquid; a barrier member arranged tosubstantially contain the liquid within the space between the projectionsystem and the substrate; a substrate table displacement system,arranged to move the substrate table along a predetermined path relativeto the barrier member, thereby moving the target portion over thesurface of the substrate; and a microwave source and microwavecontainment device, together configured to supply heat to liquid on thesurface of the substrate.

According to an aspect of the invention, there is provided alithographic apparatus, comprising: a substrate table constructed tohold a substrate; a projection system configured to project a patternedradiation beam onto a target portion of the substrate; a liquid supplysystem configured to at least partly fill a space between the projectionsystem and the substrate with liquid; a barrier member arranged tosubstantially contain the liquid within the space between the projectionsystem and the substrate; a substrate table displacement system,arranged to move the substrate table along a predetermined path relativeto the barrier member, thereby moving the target portion over thesurface of the substrate; and a substrate heater configured to heat atleast a portion of the substrate according to a position of thesubstrate table relative to the barrier member, or a velocity of thesubstrate table relative to the barrier member, or an acceleration ofthe substrate table relative to the barrier member, or a predeterminedpath of the substrate table relative to the barrier member, or a localsubstrate temperature, or a local substrate table temperature, or anycombination thereof.

According to an aspect of the invention, there is provided a devicemanufacturing method, comprising: projecting a patterned radiation beamthrough a liquid onto a target portion of a substrate; and controlling anet rate of evaporation of the liquid.

According to an aspect of the invention, there is provided a devicemanufacturing method, comprising: projecting a patterned radiation beamthrough a liquid onto a target portion of the substrate; moving asubstrate table holding the substrate along a predetermined pathrelative to a seal member used to contain the liquid, thereby moving thetarget portion over a surface of the substrate; and heating liquid onthe surface of the substrate using microwaves.

According to an aspect of the invention, there is provided a devicemanufacturing method, comprising: projecting a patterned radiation beamthrough a liquid onto a target portion of a substrate; moving asubstrate table holding the substrate along a predetermined pathrelative to a seal member used to contain the liquid, thereby moving thetarget portion over a surface of the substrate; and heating at least aportion of the substrate according to a position of the substrate tablerelative to the seal member, or a velocity of the substrate tablerelative to the seal member, or an acceleration of the substrate tablerelative to the seal member, or a predetermined path of the substratetable relative to the seal member, or a local substrate temperature, ora local substrate table temperature, or any combination thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 depicts a lithographic apparatus according to an embodiment ofthe invention;

FIGS. 2 and 3 depict a liquid supply system for use in a lithographicprojection apparatus;

FIGS. 4a and 4b depict other liquid supply systems for use in alithographic projection apparatus;

FIG. 5 depicts a barrier member according to an embodiment of theinvention, showing interaction with a pressurized gas humiditycontroller, an immersion liquid temperature controller and a pressurizedgas temperature controller;

FIG. 6 depicts a barrier member, gas-shower outlet and gas-shower outletcontroller according to an embodiment of the invention;

FIG. 7 depicts a top view of a substrate table comprising a system oflocal heaters and a substrate temperature controller according to anembodiment of the invention;

FIG. 8 depicts a side view of the substrate table of FIG. 7, showingalso a plurality of temperature sensors, a substrate table pathdetermining device and substrate heaters located in the barrier member;

FIG. 9 depicts a top view of a substrate table showing the geometry of asubstrate heater arranged to dissipate more power in the lower region ofthe substrate table than in the upper region;

FIG. 10 depicts an array of individually controllable substrate heatersaccording to an embodiment of the invention;

FIG. 11 depicts a side view of the arrangement of FIG. 10, showing alsointeraction with a heater array controller and predetermined algorithminput device;

FIG. 12 depicts a partial view of a barrier member according to anembodiment of the invention, showing a thermal isolation sleeve for avacuum exhaust inlet and vacuum exhaust pipe and a barrier memberheater;

FIG. 13 depicts interaction between a barrier member and barrier membertemperature stabilizer according to an embodiment of the invention;

FIG. 14 depicts a barrier member comprising a network of fluid-carryingchannels and a fluid supply system according to an embodiment of theinvention;

FIG. 15 depicts a barrier member and substrate table comprising anetwork of fluid-carrying channels and an array of individuallycontrollable heaters controlled by a substrate temperature controllercomprising a substrate table heat-exchange fluid controller and asubstrate heater controller;

FIG. 16 depicts a substrate table with a network of channels andcircular groove according to an embodiment of the invention;

FIGS. 17 and 18 depict the substrate table with circular grooveaccording to FIG. 16, sealed by a sealing ring according to anembodiment of the invention;

FIG. 19 depicts a lithographic apparatus showing positioning oftemperature sensors in the substrate table and barrier member accordingto an embodiment of the invention;

FIG. 20 depicts an enlarged view of the substrate table in the region ofthe substrate showing an arrangement of miniature temperature controlsystems according to an embodiment of the invention;

FIG. 21 depicts a projection system controller andthermally-induced-distortion calculator according to an embodiment ofthe invention;

FIG. 22 depicts a microwave source and microwave containment cage forheating immersion liquid on the substrate surface;

FIG. 23 depicts an arrangement of resistive heating strips andassociated electrical current flow;

FIG. 24 depicts a single resistive strip used as a local temperaturesensor for a system of local heaters;

FIG. 25 depicts an arrangement for inductive heating of the substratetable WT;

FIG. 26 depicts apparatus for producing a flow of gas with a controlledlevel of humidity;

FIG. 27 depicts a heat exchanger for controlling the temperature of agas flow; and

FIG. 28 depicts a venting system for enabling stable operation of ahumidifier cabinet.

DETAILED DESCRIPTION

FIG. 1 schematically depicts a lithographic apparatus according to oneembodiment of the invention. The apparatus comprises:

-   -   an illumination system (illuminator) IL configured to condition        a radiation beam PB (e.g. UV radiation or DUV radiation).    -   a support structure (e.g. a mask table) MT constructed to        support a patterning device (e.g. a mask) MA and connected to a        first positioner PM configured to accurately position the        patterning device in accordance with certain parameters;    -   a substrate table (e.g. a wafer table) WT constructed to hold a        substrate (e.g. a resist-coated wafer) W and connected to a        second positioner PW configured to accurately position the        substrate in accordance with certain parameters; and    -   a projection system (e.g. a refractive projection lens system)        PL configured to project a pattern imparted to the radiation        beam PB by patterning device MA onto a target portion C (e.g.        comprising one or more dies) of the substrate W.

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, fordirecting, shaping, or controlling radiation.

The support structure supports, i.e. bears the weight of, the patterningdevice. It holds the patterning device in a manner that depends on theorientation of the patterning device, the design of the lithographicapparatus, and other conditions, such as for example whether or not thepatterning device is held in a vacuum environment. The support structuremay use mechanical, vacuum, electrostatic or other clamping techniquesto hold the patterning device. The support structure may be a frame or atable, for example, which may be fixed or movable as required. Thesupport structure may ensure that the patterning device is at a desiredposition, for example with respect to the projection system. Any use ofthe terms “reticle” or “mask” herein may be considered synonymous withthe more general term “patterning device.”

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a radiation beamwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the radiation beam may not exactly correspond to the desiredpattern in the target portion of the substrate, for example if thepattern includes phase-shifting features or so called assist features.Generally, the pattern imparted to the radiation beam will correspond toa particular functional layer in a device being created in the targetportion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which may be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam which is reflected by the mirrormatrix.

The term “projection system” used herein should be broadly interpretedas encompassing any type of projection system, including refractive,reflective, catadioptric, magnetic, electromagnetic and electrostaticoptical systems, or any combination thereof, as appropriate for theexposure radiation being used, or for other factors such as the use ofan immersion liquid or the use of a vacuum. Any use of the term“projection lens” herein may be considered as synonymous with the moregeneral term “projection system”.

As here depicted, the apparatus is of a transmissive type (e.g.employing a transmissive mask). Alternatively, the apparatus may be of areflective type (e.g. employing a programmable mirror array of a type asreferred to above, or employing a reflective mask).

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables (and/or two or more support structures). In such“multiple stage” machines the additional tables may be used in parallel,or preparatory steps may be carried out on one or more tables while oneor more other tables are being used for exposure. The substrate W may beheld directly by the substrate table WT (sometimes referred to as amirror block) and may be held by a substrate holder (sometimes referredto as a burl plate or chuck), which is in turn held by the substratetable WT.

Referring to FIG. 1, the illuminator IL receives a radiation beam from aradiation source SO. The source and the lithographic apparatus may beseparate entities, for example when the source is an excimer laser. Insuch cases, the source is not considered to form part of thelithographic apparatus and the radiation beam is passed from the sourceSO to the illuminator IL with the aid of a beam delivery system BDcomprising, for example, suitable directing mirrors and/or a beamexpander. In other cases the source may be an integral part of thelithographic apparatus, for example when the source is a mercury lamp.The source SO and the illuminator IL, together with the beam deliverysystem BD if required, may be referred to as a radiation system.

The illuminator IL may comprise an adjuster AM for adjusting the angularintensity distribution of the radiation beam. Generally, at least theouter and/or inner radial extent (commonly referred to as σ-outer andσ-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator can be adjusted. In addition, the illuminator IL maycomprise various other components, such as an integrator IN and acondenser CO. The illuminator may be used to condition the radiationbeam, to have a desired uniformity and intensity distribution in itscross-section.

The radiation beam PB is incident on the patterning device (e.g., mask)MA, which is held on the support structure (e.g., mask table) MT, and ispatterned by the patterning device. Having traversed the patterningdevice, the radiation beam PB passes through the projection system PL,which focuses the beam onto a target portion C of the substrate W. Withthe aid of the second positioner PW and position sensor IF (e.g. aninterferometric device, linear encoder or capacitive sensor), thesubstrate table WT may be moved accurately, e.g. so as to positiondifferent target portions C in the path of the radiation beam PB.Similarly, the first positioner PM and another position sensor (which isnot explicitly depicted in FIG. 1) may be used to accurately positionthe patterning device MA with respect to the path of the radiation beamPB, e.g. after mechanical retrieval from a mask library, or during ascan. In general, movement of the support structure MT may be realizedwith the aid of a long-stroke module (coarse positioning) and ashort-stroke module (fine positioning), which form part of the firstpositioner PM. Similarly, movement of the substrate table WT may berealized using a long-stroke module and a short-stroke module, whichform part of the second positioner PW. In the case of a stepper (asopposed to a scanner) the support structure MT may be connected to ashort-stroke actuator only, or may be fixed. Patterning device MA andsubstrate W may be aligned using patterning device alignment marks M1,M2 and substrate alignment marks P1, P2. Although the substratealignment marks as illustrated occupy dedicated target portions, theymay be located in spaces between target portions (these are known asscribe-lane alignment marks). Similarly, in situations in which morethan one die is provided on the patterning device MA, the patterningdevice alignment marks may be located between the dies.

The depicted apparatus could be used in at least one of the followingmodes:

1. In step mode, the support structure MT and the substrate table WT arekept essentially stationary, while an entire pattern imparted to theradiation beam is projected onto a target portion C at one time (i.e. asingle static exposure). The substrate table WT is then shifted in the Xand/or Y direction so that a different target portion C can be exposed.In step mode, the maximum size of the exposure field limits the size ofthe target portion C imaged in a single static exposure.

2. In scan mode, the support structure MT and the substrate table WT arescanned synchronously while a pattern imparted to the radiation beam isprojected onto a target portion C (i.e. a single dynamic exposure). Thevelocity and direction of the substrate table WT relative to the supportstructure MT may be determined by the (de-) magnification and imagereversal characteristics of the projection system PL. In scan mode, themaximum size of the exposure field limits the width (in the non-scanningdirection) of the target portion in a single dynamic exposure, whereasthe length of the scanning motion determines the height (in the scanningdirection) of the target portion.

3. In another mode, the support structure MT is kept essentiallystationary holding a programmable patterning device, and the substratetable WT is moved or scanned while a pattern imparted to the radiationbeam is projected onto a target portion C. In this mode, generally apulsed radiation source is employed and the programmable patterningdevice is updated as required after each movement of the substrate tableWT or in between successive radiation pulses during a scan. This mode ofoperation can be readily applied to maskless lithography that utilizesprogrammable patterning device, such as a programmable mirror array of atype as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

A further immersion lithography solution with a localized liquid supplysystem is shown in FIG. 4a . Liquid is supplied by two groove inlets INon either side of the projection system PL and is removed by a pluralityof discrete outlets OUT arranged radially outwardly of the inlets IN.The inlets IN and OUT can be arranged in a plate with a hole in itscenter and through which the projection beam is projected. Liquid issupplied by one groove inlet IN on one side of the projection system PLand removed by a plurality of discrete outlets OUT on the other side ofthe projection system PL, causing a flow of a thin film of liquidbetween the projection system PL and the substrate W. The choice ofwhich combination of inlet IN and outlets OUT to use can depend on thedirection of movement of the substrate W (the other combination of inletIN and outlets OUT being inactive).

Another solution which has been proposed is to provide the liquid supplysystem with a barrier member which extends along at least a part of aboundary of the space between the final element of the projection systemand the substrate table. The barrier member is substantially stationaryrelative to the projection system in the XY plane though there may besome relative movement in the Z direction (in the direction of theoptical axis). In an embodiment, a seal is formed between the barriermember and the surface of the substrate. The seal may be a contactlessseal such as a gas seal. Such a system is disclosed in United Statespatent application publication no. US 2004-0207824 and European patentapplication publication no. EP 1420298, each hereby incorporated in itsentirety by reference.

FIG. 4b shows an example arrangement for a barrier member 12 configuredto contain an immersion liquid in the localized area 25 under theprojection system PL. The barrier member 12 is provided with anextractor EX configured to extract liquid from the localized area 25through a gauze GZ. The extractor EX may extract both liquid and gas oronly liquid. A recess RE is provided radially outwardly of the extractorEX and a gas seal 27 is provided radially outwardly of the recess RE.The gas seal 27 forms a jet of gas JE which is used to dry the surfaceof the substrate W and/or reduce the amount of liquid that escapes fromthe barrier member 12.

According to an aspect of the invention, overlay errors and otherproblems linked to the presence of an immersion liquid and barriermember 12 may be reduced by a liquid evaporation controller, whichtargets and controls the rate of evaporation of immersion liquid in theregion of the substrate. Molecules of liquid absorb energy from thesurroundings in order to evaporate and, particularly if pumped away, thecooling that results may result in significant and non-uniformvariations in the temperature of critical components such as thesubstrate W. Thermally-induced distortions may lead to errors in theimage finally written to the substrate. For example, evaporation ofimmersion liquid left behind on the substrate after the barrier member12 has passed by may cause local temperature drops of up to 3K. As aresult of this, over 20 nm single machine overlay error might result.

FIG. 5 shows an arrangement of the barrier member 12 according to anembodiment of the invention. Immersion liquid is contained within animmersion reservoir 25 located between the final element of theprojection system PL and the substrate W. Immersion liquid is containedwithin the immersion reservoir 25 by the body of the barrier member 12and by a gas seal 27 on its lower periphery, which limits the amount ofimmersion liquid that escapes from the immersion reservoir 25 throughgap 22. The gas seal 27 is connected to a pressurized gas supply system30, which supplies pressurized gas to the gas seal 27 via a pressurizedgas outlet and a pressurized gas supply pipe 15. Gas is pumped away viaa vacuum exhaust inlet and a vacuum exhaust pipe 14. Immersion liquidthat evaporates in the region of the gas seal 27 may be pumped away viathe vacuum exhaust inlet 17. Alternatively, liquid that escapes beyondthe gas seal 27 either into the region underneath the barrier member 12in the gap 22 or beyond the outer edges of the barrier member 12, mayevaporate into the environment outside the substrate W outside of thebarrier member 12.

Where a substance exists both in liquid form and in gaseous form, it isnormally the case that a dynamic equilibrium will exist with a rate ofevaporation of liquid being balanced against a rate of condensation ofvapor. The amount of cooling caused by evaporation will therefore beoffset by a heating caused by condensation (where high energy gaseousmolecules yield energy to their surroundings as part of the transitionto the lower energy liquid state). The cooling power therefore dependson the net rate of evaporation (i.e. the difference between the numberof molecules entering the gaseous state from the liquid per unit timeand the number of molecules entering the liquid state from the gaseousstate per unit time). Both condensation and evaporation are statisticaleffects and increasing the number of molecules involved will increasethe rate of either process. Therefore, increasing the vaporconcentration will increase the rate of condensation and lead to areduction in the net rate of evaporation. Where the vapor consists ofwater molecules, this concentration is directly related to the relativehumidity, defined as the amount of water vapor present as a percentageof the maximum quantity that could be present at a given temperature.

This understanding is exploited according to an embodiment of theinvention to control the cooling caused by evaporation of immersionliquid. As illustrated in FIG. 5, a pressurized gas humidity controller50, which is provided configured to interact with the pressurized gassupply system 30 to provide pressurized gas to the gas seal 27 with arelative humidity controlled to be greater than about 10%. Increasingthe relative humidity of the gas increases the rate of condensation andtherefore decreases the net rate of evaporation and the cooling causedthereby. In an embodiment, the relative humidity is arranged to bewithin a predetermined range determined by reference to calibrationmeasurements. For the purposes of controlling cooling, in general, thehigher the relative humidity the better. However, for very high relativehumidities, the barrier member 12 may leave excessive quantities ofwater in its wake. Furthermore, if insufficient mechanisms are providedfor humid gas extraction near the outer diameter of the barrier member,humid gas may leave and interfere with the operation of position sensorIF. Therefore, the upper limit will in general be dependent on detailsof the barrier member construction and/or configuration. Additionally oralternatively, the predetermined range may be greater than about 40%.High relative humidities such as these may be achieved by using lowerworking pressures than might otherwise be selected for the purposes ofachieving optimal sealing properties (6 bar may typically be used).Ideally, a working pressure should be chosen that is as close aspossible to atmospheric pressure, while still providing sufficient flowrate for the gas seal 27 to perform its function. The lower the workingpressure, the less the relative humidity will be reduced when thepressurized gas expands on leaving the pressurized gas supply system 30.

The pressurized gas humidity controller 50 may be arranged to beresponsive to changes in the temperature of the substrate W and/orsubstrate table WT. These temperature changes may be determined via oneor more temperature sensors 60 arranged, for example, in the substratetable WT. According to an embodiment of the invention, the pressurizedgas humidity controller 50 is arranged to compare the temperatures ofthe substrate W and/or substrate table WT and/or substrate holder,measured at one or more points by the temperature sensor(s) 60, with atarget temperature or temperatures Tt. That is to say, where a singletemperature sensor 60 is present, the pressurized gas humiditycontroller 50 compares this one temperature reading with a single targettemperature Tt. Where a plurality of temperature sensors 60 are present,the pressurized gas humidity controller 50 compares a plurality ofreadings with a single target temperature Tt, or with a plurality oftarget temperatures Tt corresponding, for example, to particular regionsof the substrate W and/or corresponding regions of the substratetable/substrate holder, and therefore to particular groups oftemperature sensor readings (within which an average reading may beused). The pressurized gas humidity controller 50 may then adjust therelative humidity of the pressurized gas in order to reduce a differencebetween the measured and target temperature(s), the efficiency of theprocess may be controlled by a feedback controller such as a PID system.

Adjusting the humidity of the gas supplied to the gas seal 27 may bemost efficient for cooling caused by evaporation in the region of thegas seal 27, particularly around the vacuum exhaust inlet 18 and vacuumexhaust pipe 14. In an embodiment, it is desirable to have an additionalmechanism to control the net evaporation of liquid beyond the gas seal27 and outside of the barrier member 12. Such an arrangement, accordingto an embodiment of the invention, is illustrated in FIG. 6. Here, agas-shower outlet 70 is provided that is capable of providing a flow ofgas with a relative humidity controlled to be greater than 10%. Agas-shower humidity controller 75 is provided that is capable ofadjusting the relative humidity in accordance either with calibrationmeasurements, calculations or measurements of the temperature at one orvarious points on the substrate W and/or substrate table/substrateholder, as provided by one or more temperature sensors 60, and comparingthem with a target temperature or temperatures Tt. A desired range ofrelative humidity in this case is about 40 to about 50%. In the casewhere the gas-shower humidity controller 75 is arranged to respond totemperature measurements, it may adjust the relative humidity of the gasin order to reduce a difference or differences between the measured andtarget temperature(s) Tt. That is to say, where a single temperaturesensor 60 is present, the gas-shower humidity controller 75 comparesthis one temperature reading with a single target temperature Tt. Wherea plurality of temperature sensors 60 are present, the gas-showerhumidity controller 75 compares a plurality of readings with a singletarget temperature Tt, or with a plurality of target temperatures Ttcorresponding, for example, to particular regions of the substrate Wand/or corresponding regions of the substrate table/substrate holder,and therefore to particular groups of temperature sensor readings(within which an average reading may be used). A feedback controller,such as a PID system, may control the efficiency of the process.

The gas-shower humidity controller 75 may be arranged to interact withthe pressurized gas humidity controller 50 so as to ensure that therelative humidity of the gases provided by the gas seal 27 and thegas-shower outlet 70 are matched. This feature provides a mechanism bywhich variations in the relative humidity outside of the gas seal 27 maybe controlled and avoids disturbances to systems, such as theinterferometers used to measure the position of the substrate table WT,that might otherwise occur.

The substrate table WT is normally arranged to be moved relative to theprojection system PL and barrier member 12 by a substrate tabledisplacement system 100 (see FIG. 8), so that consecutive target regionsof the substrate W may be exposed by the patterned radiation beam. Thisprocess may encourage small quantities of immersion liquid to leave theconfines of the immersion reservoir 25 in spite of the operation of thegas seal 27. Embodiments arranged to reduce component cooling caused byreducing immersion liquid evaporation have been discussed above.According to an alternative aspect of the invention, errors caused bythe cooling effects of evaporating immersion liquid may be reduced byproviding a substrate heater, configured to heat at least a portion ofthe substrate W according to the position, velocity, acceleration, orpredetermined path of the substrate table WT relative to the barriermember 12, the local substrate W, the substrate table WT temperature, orany combination thereof. The substrate heater may provide heating via anumber of mechanisms. These may include: an infra-red emission source, aglow wire electrical resistance heater, a hot gas jet, or anycombination thereof. Factors when deciding which kind of heater to usemay include how finely and quickly the heating power needs to beadjusted, as well as how effectively the heater can be produced inminiature form. The latter factor may become more or less importantdepending on whether the heater needs to be embedded in or near thematerial whose temperature it is intended to regulate (such as a glowwire, for example, embedded in the substrate table WT), or whether theheater works to some extent at a distance (such as a radiation-emittingsource or temperature-controlled gas jet source). In the case of aradiation-emitting source, the wavelength distribution of the radiationshould be chosen so as not to react with resist compositions on thesubstrate W (infra-red radiation would be safe in this regard for mostresists of interest). Selection of radiation intensity will depend onthe optical properties of the resist (such as its reflectivity). Thismay be determined by calibration measurements during a set-up sequenceof the lithography apparatus. Where there is likely to be a processstage dependency (due to variations in the reflectivity, for example),calibration may also be carried out within the production sequence as anextra measurement phase for each lot of substrates. As shall bedescribed below, several embodiments of the invention operate on theprinciple that at least a subset of the substrate heaters present areactuated during a substrate exposure sequence, i.e. as the barriermember 12 passes over the substrate W. However, a system that heats thesubstrate W before exposure, so as to compensate for cooling that hasyet to occur but is expected to occur, also falls within the scope ofthe invention.

FIGS. 7 and 8 show an arrangement according to an embodiment of theinvention, comprising a system of heaters 85,86 embedded either in thesubstrate table as “local heaters” 85, or in the barrier member 12 as“remote heaters” 86, or both. The local heaters 85 are each arranged toheat predominantly a particular region of the substrate W and togethermay be used to control the temperature profile of at least a portion ofthe substrate W. The remote heaters 86 will heat a different portion ofthe substrate W depending on the position of the barrier member 12relative to the substrate W.

According to a first mode of operation involving the local heaters 85,the heating power and relative timing of each heater may be adjusted toset up a starting temperature profile for the substrate W at a knownperiod of time before the substrate exposure cycle is started. Byreference to calibration measurements and/or analysis of test patternsgenerated by the lithographic apparatus, a starting temperature profilemay be selected that substantially compensates for cooling that willoccur due to evaporation of immersion liquid during the exposure cycle.

According to a second mode of operation involving the local heaters 85,each of these heaters 85 may be arranged to be switched to aheat-emitting state only when the barrier member 12 passes over theregion that they are positioned to heat. For example, in the case wherethe barrier member 12 (and thus the target area) moves relative to thesubstrate W as shown in FIG. 7, along a path 150 between an initial die(or target area) 160 to a final die 170, the local heaters 85 would alsobe switched on in a progressive manner substantially along the same path150. This may be achieved by programming a substrate temperaturecontroller 110 to provide a series of time-delayed actuation signalsthat for each local heater 85 closely lags behind the intended path ofthe barrier member 12 relative to the substrate table WT. The intendedpath may be stored in a substrate table path determining device 90. Asan alternative or additional approach, the actuation sequence of thelocal heaters 85 may be derived from further functions of the substratetable path determining device 90. For example, the substrate table pathdetermining device 90 may comprise a device (based on interferometry,for example) to measure the position, velocity and/or acceleration ofthe substrate table WT and to feed this information to the substratetemperature controller 110, which may be configured to calculate at thispoint when to activate each local heater 85. For example, the pathdetermining device 90 may be configured to send an actuation signal to agiven heater when it detects that the barrier member 12 is moving awayfrom, or moved past, that particular heater. The power supplied by eachlocal heater 85 may be arranged to be constant or time-varying and bethe same as or different to the other local heaters 85. The optimalarrangement to use for each heater is that which best compensates thepower lost due to evaporation for the region concerned. In the casewhere the rate of loss of liquid from the barrier member 12 is constant,the power to be supplied by each heater 85 once activated may besubstantially the same (because once the barrier member 12 has passedby, the amount of liquid left behind on the substrate W to evaporate maybe found to be roughly constant). Alternatively, more heating power maybe required in certain regions, such as where the barrier member 12changes direction relative to the substrate table WT. Calibrationmeasurements may be carried out to determine the most effective way tooperate the heater powers as a function of the particular substratetable path and rate required.

The remote heaters 86 in the barrier member 12 may, in an embodiment, bepositioned around the periphery of the barrier member 12, as shown inFIG. 8. This arrangement allows the heaters to operate in closeproximity to the region where the evaporation processes may beextracting the most heat. Placement near the outer diameter may bechosen as a compromise to avoid the region immediately around the gasseal 27, which in practice is already heavily occupied by holes, pipesand ducts. Because they operate at a distance from the substrate W,heater mechanisms such as those based on radiation or hot gas jets maybe appropriate. Creating a hot surface in the base of the barrier member12 is one method by which a radiation source may be implemented. Thermalisolation of such a device from the rest of the barrier member 12 mayimprove the performance of such a feature. Alternatively oradditionally, infra-red bulbs may be used.

As was arranged for the local heaters 85 above, the power of the remoteheaters 86 may be controlled depending on the direction of motion of thesubstrate table. For example, it may be arranged to provide more heatfrom one side of the barrier member 12 than from the other. As oneaspect of the cooling is related to evaporation of liquid escaping fromthe barrier member 12, the remote heaters 86 on the trailing edge of thebarrier member 12 (where immersion liquid is likely to be escaping) maybe configured to emit a higher heating power than those located on thefront edge of the barrier member 12 (where the substrate W is stilldry). The effectiveness of the remote heaters 86 may be varied on demandby varying the power and/or width of the heaters 86 around thecircumference of the barrier member 12. This latter parameter may bevaried, for example, by progressively activating different segments of asegmented heater 86, or one heater 86 of a plurality of heaters 86.

Although shown embedded in the substrate table WT or barrier member 12,it is to be understood that the heaters 85,86 may be positioned anywherewhere they can influence the temperature of the substrate W.Radiation-emitting heaters, for example, may be positioned in a separatebody distinct from the substrate table WT and barrier member 12. Wherethe substrate W is heated prior to exposure, this may occur in a regionaway from that used for exposure, so as to implement more easily theremote heaters 86.

The lithographic apparatus may also comprise local temperature sensors60, which in the example illustrated in FIG. 8 are embedded in thesubstrate table WT. According to an embodiment of the invention, thesetemperature sensors 60 are arranged to measure the temperature of eachregion of the substrate W and/or corresponding region of the substratetable/substrate holder affected by each local heater 85. Thisinformation is fed to the substrate temperature controller 110, whichmay then calculate how to control the output of the local heaters 85and/or remote heaters 86 so as to reduce a difference between a targettemperature or temperatures Tt and the temperatures measured by thelocal temperature sensors 60. In this embodiment, it may be desirable toarrange for the heaters 85 and/or 86 to have a variable rather thanfixed output. In either case, a feedback controller (such as PID) may beused to optimize the efficiency of the convergence process.

Modulating the temperature of the liquid supplied by the liquid supplysystem 130 may also control the temperature of the substrate W and/orsubstrate table/substrate holder. For example, the immersion liquid maybe heated to a controlled temperature greater than 295 K. FIG. 5 showsan embodiment of the invention comprising an immersion liquidtemperature controller 120 arranged to carry out this function incooperation with the liquid supply system 130. The temperature controlof the immersion liquid may be effected with reference to calibrationmeasurements or with reference to readings from one or more temperaturesensors 60 in order to select an immersion liquid temperature that willcompensate effectively for evaporation heat loss. In the latter case,the output of the immersion liquid temperature controller 120 may becontrolled so as to minimize a difference or differences between atarget temperature or temperatures Tt and the temperature(s) provided bythe temperature sensor(s) 60, the convergence process being controlledby a feedback controller, such as a PID controller. That is to say,where a single temperature sensor 60 is present, the immersion liquidtemperature controller 120 compares this one temperature reading with asingle target temperature Tt. Where a plurality of temperature sensors60 are present, the immersion liquid temperature controller 120 comparesa plurality of readings with a single target temperature Tt, or with aplurality of target temperatures Tt corresponding, for example, toparticular regions of the substrate W and/or corresponding regions ofthe substrate table/substrate holder, and therefore to particular groupsof temperature sensor readings (within which an average reading may beused).

Modulating the temperature of the gas supplied by the pressurized gassupply system 30 may also control the temperature of the substrate Wand/or substrate table/substrate holder. For example, the pressurizedgas may be heated to a controlled temperature greater than about 300 K.The lower temperature limit here is higher than that required for theimmersion liquid temperature controller 120 described above due to thelower heat capacity of the gas relative to the liquid. According to oneembodiment of the invention, pressurized gas is provided at temperaturesin the range of about 300 to about 320 K. FIG. 5 shows an embodiment ofthe invention comprising a pressurized gas temperature controller 140arranged to carry out the temperature control function in cooperationwith the pressurized gas supply system 30. The temperature control ofthe pressurized gas may be effected with reference to calibrationmeasurements or with reference to readings from one or more temperaturesensors 60. In the latter case, the output of the pressurized gastemperature controller 140 may be controlled so as to minimize adifference or differences between a target temperature or temperaturesTt and the temperature(s) provided by the temperature sensors 60, theconvergence process being controlled by a feedback controller, such as aPID controller. That is to say, where a single temperature sensor 60 ispresent, the pressurized gas temperature controller 140 compares thisone temperature reading with a single target temperature Tt. Where aplurality of temperature sensors 60 are present, the pressurized gastemperature controller 140 compares a plurality of readings with asingle target temperature Tt, or with a plurality of target temperaturesTt corresponding, for example, to particular regions of the substrate Wand/or corresponding regions of the substrate table/substrate holder,and therefore to particular groups of temperature sensor readings(within which an average reading may be used).

As has already been discussed, the substrate heating requirements have apositional dependence that may be at least in part determined by thepath of the barrier member 12 over the substrate W. At least twoprocesses have been identified as contributing to the cooling process:evaporation of liquid in the gap 22 between the substrate W and thebarrier member 12, and evaporation of residual liquid left on thesubstrate W after exposure, if the exposed area is left wet. The coolingpower of the barrier member 12 (i.e. cooling from the first process) isconstant in time although it depends, inter alia, on the velocity of thebarrier member 12 relative to the substrate W. The cooling power of thesecond process depends, inter alia, on the amount of liquid left on thesubstrate W. The amount of cooling that needs to be compensated is ingeneral a complex function of both processes, leading to a cooling powerwith a complex positional dependence. Thermal conduction within thesubstrate W will also be a factor, meaning that unexposed areas of thesubstrate W may start to cool even before the barrier member 12 reachesthem due to cooling in exposed portions of the substrate W.

Taking one process at a time, however, it is possible to make someestimations. For example, considering only the direct cooling fromevaporation of residual immersion liquid on the substrate W, a substrateexposure configured to take about 30 seconds, with a time between a lastexposure and substrate unloading of about 5 seconds, and an exposuresequence such as that labeled 150 in FIG. 7, about 20% to about 30% moreheat may be expected to be extracted by this mechanism at the location160 of the first exposure than at location 170 of the last exposure. Incertain embodiments described above comprising substrate heaters 85,86,this effect was taken into account by delaying the action of individualheaters along the path of the barrier member 12. A similar effect mayalso be achieved by configuring the substrate heater to provide a higherheating power at target regions on the substrate W where the projectionsystem is configured to project the patterned radiation beam at a firsttime and provide progressively lower heating powers at target regions onthe substrate W where the projection system PL is configured to projectthe patterned radiation beam at later times. This arrangement may bevaried to give a more complex position-dependent heating depending onthe cooling characteristics of a particular setup that have to becompensated.

Although it may be technically possible to position a large number oflocal heaters 85 at many different locations on the substrate W, it is,in practice, almost as effective and significantly less expensive toprovide a more limited number of heaters and position them tosubstantially follow the path of the barrier member 12. An arrangementof this type is depicted in FIG. 7. Here, elongate substrate heaters 85,such as glow wires, are arranged so that one individually controllableelement is aligned with one of the principal scanning or stepping axes181-187 (each corresponding to a row of dies) of the barrier member 12over the substrate W. In the example shown, each glow wire 85 isarranged to emit a constant heating power per unit length and isconfigured so that the glow wire aligned with the principal scanning orstepping axis 187 has the highest heating power, that aligned with 186the next highest heating power and so on, progressively decreasing untilthe final glow wire, corresponding to principal scanning or steppingaxis 181, is reached, which is assigned the lowest heating power.

Where a large number of local heaters are provided at differentlocations (for example, between 100 and 700 per substrate), it isdesirable to position the heaters as close as possible to the surface ofthe substrate W. In the arrangement shown in FIGS. 7 and 8, however,where fewer heaters are provided, it is desirable to position theheaters substantially further away so that each heater may haveeffective control over a larger portion of the substrate W.

FIG. 9 shows an arrangement wherein a continuous glow wire heater 85 isprovided to heat the substrate W. In the example shown, the glow wireheater 85 is arranged to follow the path of the barrier member 12 tosome extent in that it has longer sections 195 substantially parallel tothe principal scanning axes 181-187 (i.e. perpendicular to the scanningdirection) of the barrier member 12 (as shown in FIG. 7). However, thepitch 191-193 between these longer sections is arranged to becomeshorter and shorter towards the lower end of the substrate W asdepicted, which corresponds to the region within which the substrate Wwill be first exposed (i.e. pitch 191>pitch 192>pitch 193). This meansthat the glow wire heater 85 may be given the simplest and most robustconstruction (that where the heating power is constant per unit length,which in practice may correspond to an elongate resistive element ofconstant cross-section) and yet still provide a heating power that islarger towards the region within which the substrate W will be firstexposed, which is the region of the substrate W that will require thelargest correction for cooling effects. As an alternative and/oradditional arrangement, the glow wire heater 85 may be arranged toprovide a heating power per unit length that varies along its length(for example, increasing towards the lower end of the substrate W in theorientation depicted). In the case where the glow wire operates by meansof electrical dissipation associated with an electric current passedalong its length, variable heating power may be achieved either byvarying the cross-section (for example, providing a glow wire thatbecomes thinner where more power is needed) or by varying the materialused. In the latter arrangement, care should be taken to avoid points ofhigh resistance where junctions are made between materials of differentcomposition.

FIGS. 10 and 11 show an arrangement wherein the substrate heatercomprises a system of individually controllable heaters 85. In theembodiment shown in FIG. 10, the individually controllable heaters 85are arranged as elongate members substantially parallel to the principalscanning axes 181-187 (i.e. perpendicular to the scanning direction) andconfined to heat within the geometrical limits of the substrate W.However, alternative arrangements of heaters would also be compatiblewith this embodiment of the invention as long as they may be controlledindividually. A heater array controller 180 controls each individuallycontrollable heater 85 via an address bus. The heater array controller180 receives in turn input from a predetermined algorithm 190, whichdescribes how the heating power of each individual heater should becontrolled as a function of time (and therefore as a function of theposition of the barrier member 12 relative to the individual heater inquestion). The appropriate algorithm to use may be derived fromcalibration measurements and/or calculation (based, for example, on theamount of time an expected amount of liquid is expected to remain on thesubstrate W). This approach has the advantage of not requiringtemperature sensors, which may greatly simplify construction.

Evaporation of immersion liquid may also lead to cooling of the barriermember 12 itself. This effect may lead in turn to cooling of thesubstrate W, for example, by cooling the immersion liquid and/orpressurized gas by convection and/or by radiative effects. According toan aspect of the invention, a barrier member temperature stabilizer isprovided to reduce cooling of the substrate W by this mechanism.

A region of particular concern is around the vacuum exhaust inlet 17 andin the vacuum exhaust pipe 14, as shown in FIG. 12. Where immersionliquid is present in these regions, net evaporation may be particularlypronounced, as the concentration of vapor may be kept low by the vacuumsystem (evaporated liquid will immediately be pumped away). One way inwhich overall cooling of the barrier member 12 due to this mechanism maybe controlled is illustrated in FIG. 12, where the barrier membertemperature stabilizer is effected by means of a thermal isolationsleeve 210 arranged around the vacuum exhaust pipe 14. In an embodiment,the thermal isolation sleeve 210 should be formed from a material havingvery low thermal conductivity at the expected operating temperature ofthe lithographic apparatus. General purpose plastics, PTFE, etc. may beappropriate materials for the thermal isolation sleeve 210.Alternatively or additionally, the barrier member itself may beconstructed wholly or partially of a thermally isolating material. Thisapproach may be more effective and more easily implemented than havingonly a thermal isolation sleeve 210, although the choice of materialswith suitable mechanical characteristics may be restricted.

An additional and/or alternative approach is to provide a dedicatedbarrier member heater 220, which is arranged to provide a compensatingheating power to those areas of the barrier member 12 cooled byevaporation of the immersion liquid. Although directed on the one handto heating the barrier member 12 itself, and thus, indirectly, thesubstrate W, the barrier member heater 220 may be arranged to heat thesubstrate W directly. This may be achieved through the use of aradiation-emitting heater such as an infra-red heater, which has beendescribed above in the context of possible substrate heaters 85,86. Inthe arrangement shown in FIG. 12, the barrier member heaters 220 arearranged around the vacuum exhaust inlet 17 and may follow the geometryof the vacuum exhaust inlet in a plane perpendicular to the axis of thebarrier member 12 (into the page in the orientation depicted).

The heating power of the barrier member heater 220 may be controlled bythe barrier member temperature stabilizer according to input from one ormore of a number of possible sources. For example, the barrier memberheater power may be adjusted in response to the flow rate in the vacuumexhaust pipe 14, which may be provided by the pressurized gas supplysystem 30. Here, it is expected that a higher heating power may berequired for higher flow rates.

The barrier member heater 220 may also be controlled by reference to thetemperature of the substrate W and/or substrate table/substrate holder,which may be measured at one or more positions by one or moretemperature sensors 60. As with previous embodiments, a feedbackcontroller may be employed to reduce a difference between the measuredsubstrate temperature(s) and one or more predefined target temperaturesTt.

The barrier member heater 220 may also be controlled in response to therelative humidity of the gas supplied by the pressurized gas outlet 18.This information may be provided by humidity sensors, which may bearranged either in the barrier member 12 or as part of the pressurizedgas supply system 30 (the latter case being illustrated in FIG. 13).

Finally, the barrier member temperature stabilizer 200 may control theoutput of the barrier member heater 220 by reference to a calibrationtable 230 of required corrections, constructed from measurements of thebarrier member temperature as a function of: substrate temperature,pressurized gas flow rate, pressurized gas flow temperature, vacuumexhaust flow rate, vacuum exhaust temperature, pressurized gas relativehumidity, immersion liquid temperature, or any combination thereof.Although calibration measurements should be undertaken, this approachgreatly reduces the need to incorporate additional functional componentsin the final lithographic apparatus to be shipped to the customer.

When considering the problem of substrate cooling linked to a cooledbarrier member 12, an important region of the barrier member 12 is thatclosest to and/or facing the substrate W. According to an embodiment ofthe invention depicted in FIG. 14, the barrier member 12 is constructedwith a network of channels distributed in a layer 400 in the portion ofthe barrier member 12 closest the substrate W. The barrier membertemperature stabilizer 200 is configured to control a heat-exchangefluid supply system 410 that provides a heat-exchange fluid to thenetwork at a controlled temperature and/or flow rate. As in previousembodiments, a feedback controller may be provided to help control thesubstrate temperature in an efficient manner. In this case, thetemperature and/or flow rate of the heat-exchange fluid provided by thefluid supply system 400 may be adjusted so as to reduce a differencebetween one or more substrate temperatures and/or substrate tabletemperatures, as measured by a system of local temperature sensors 60,and a target temperature or temperatures Tt. That is to say, where asingle temperature sensor 60 is present, the fluid supply system 400compares this one temperature reading with a single target temperatureTt Where a plurality of temperature sensors 60 are present, the fluidsupply system 400 compares a plurality of readings with a single targettemperature Tt, or with a plurality of target temperatures Ttcorresponding, for example, to particular regions of the substrate Wand/or corresponding regions of the substrate table/substrate holder,and therefore to particular groups of temperature sensor readings(within which an average reading may be used). The temperature and/orflow rate of the fluid may also be controlled by reference to acalibration table 230 of required corrections, constructed frommeasurements of the barrier member temperature as a function of:substrate temperature, pressurized gas flow rate, pressurized gas flowtemperature, vacuum exhaust flow rate, vacuum exhaust temperature,pressurized gas relative humidity, immersion liquid temperature, or anycombination thereof. Although calibration measurements should beundertaken, this approach greatly reduces the need to incorporateadditional functional components in the final lithographic apparatus tobe shipped to the customer.

A general advantage of those embodiments described above that depend onmechanisms located in the barrier member 12 is that they may beimplemented without influencing the dynamic performance of the substratetable WT (this may be true both for fluid-based and electrical systems).Barrier member temperature conditioning also improves not onlyshort-term (die to die) temperature variations in the substrate W, butalso long-term temperature variations from one substrate W to the next.More generally, the development costs (and development time) associatedwith barrier member improvements are likely to be significantly lowerthan those involving the substrate table WT. Apart from the problemsassociated with controlling the dynamics of the substrate table WT, afurther factor in favor of working on the barrier member 12 rather thanthe substrate table WT relates to the flatness requirements, which areapproximately 100 times more relaxed for the barrier member 12. This maybe important, for example, where channels are machined into the barriermember 12. Introducing holes near the surface (where they are mosteffective) tends to introduce surface irregularities (bulges) as mightvariations in the pressure of the heat-exchange fluid (due to thereduced stiffness of the thin width of material left between the outersurface of the barrier member and the internal channel edge.

FIGS. 15 to 18 depict an arrangement also comprising a network offluid-carrying channels. In this arrangement, the channels are locatedin the substrate table WT in close proximity to the substrate W. Thisarrangement of channels is configured to control the temperature of thesubstrate W, which may be adversely affected by evaporation of immersionliquid from its top surface.

In the present embodiment, a substrate table heat-exchange fluidcontroller 510 is provided for controlling the temperature and flow rateof a heat-exchange fluid arranged to flow through the network ofchannels 500.

As in previous embodiments, a feedback controller may be provided tohelp control the substrate temperature in an efficient manner. In thiscase, the temperature and/or flow rate of the substrate tableheat-exchange fluid may be adjusted so as to reduce a difference betweenone or more substrate temperatures and/or substrate table/substrateholder temperatures, as measured by a system of local temperaturesensors 60, and a target temperature or temperatures Tt.

The arrangement may work particularly effectively if local substrateheaters, such as glow wires, are also included so as to implement a“push-pull” principle of temperature control. According to thisembodiment, a substrate temperature controller 520 controls theoperation of a substrate heater controller 430 and the substrate tableheat-exchange fluid controller 510. A feedback controller may beincluded as part of the substrate temperature controller 520, arrangedto minimize a difference or differences between the substratetemperature, as measured at one or more locations on the substrate Wand/or substrate table/substrate holder by local temperature sensors 60,and a target temperature or temperatures Tt. That is to say, where asingle temperature sensor 60 is present, the substrate temperaturecontroller 520 compares this one temperature reading with a singletarget temperature Tt. Where a plurality of temperature sensors 60 arepresent, the substrate temperature controller 520 compares a pluralityof readings with a single target temperature Tt, or with a plurality oftarget temperatures Tt corresponding, for example, to particular regionsof the substrate W and/or corresponding regions of the substratetable/substrate holder, and therefore to particular groups oftemperature sensor readings (within which an average reading may beused). Alternatively, a feedforward loop may be used if heat flows canbe calculated as a function of the velocity and position of the barriermember 12 relative to the substrate W. According to the “push-pull”principle, the heat-exchange fluid controller 510 may be arranged toprovide fluid at a temperature below the target temperature Tt,effectively acting to cool the substrate W. The local substrate heaters,which may be electrical resistance heaters (glow wires) as mentionedabove, may respond much more quickly to sudden increases in theevaporation rate than can the heat-exchange fluid controller. Theirresponse speed is additionally improved by being set against the coolingaction of the heat-exchange fluid controller. Furthermore, shouldovershoot of the substrate temperature occur, the provision of thecooling heat-exchange fluid may allow a more rapid return to equilibriumthan would be the case if no additional cooling mechanism was provided.

For ease of machining (among other reasons), the network of channels 500comprise an array of substantially straight holes (which may be drilled)oriented in the plane of the substrate table, as depicted in FIG. 16.The ends of these straight holes should be connected and closedwatertight. This may be done with plugs glued in the holes. However, ina typical configuration comprising about 4 mm holes on an about 8 mmpitch, more than 80 plugs may be required. In addition to the problemthat many individual elements need to be constructed, the possibilityexists in such an arrangement that dead ends occur in which either nofluid arrives at all, or fluid is not circulated. According to anembodiment of the invention, these problems may be overcome by providinga circular groove 420 (shown in FIGS. 16 to 18), at the edge of thesubstrate table WT into which all through holes can be connected in aneat fashion without dead ends. This arrangement may have a furtheradvantage in that fluid may be made to circulate closer to the edge ofthe substrate table WT. The circular groove 420 may be sealed using amuch smaller number of components. In the embodiment shown, a sealingring 410 is used, which may be split into two components for ease ofassembly and attached to the groove by glue or some other standardbonding technique. The improved fluid distribution provides a more evenand controlled cooling for the substrate table WT, allowing moreeffective thermal management and improved overlay.

In the above-described embodiments, local substrate temperature sensors60, where included, have been shown embedded in the substrate table WTclose to the substrate W. These sensors may operate on a variety ofprinciples based generally on the measurement of a calibrated andreproducible temperature-dependent property (such as electricalresistance). Although shown embedded in the substrate, the local sensorsmay also be positioned in the barrier member 12 as shown in FIG. 19. Asthe thermal connection is relatively poor across the gap 22 (unlike thatbetween the substrate W and sensors when embedded in the substrate tableWT, where a high thermal conductance can more easily be arranged), it isdesirable that sensors 60 located in the barrier member 12 operate byanalyzing radiation emitted from the substrate W. According to anembodiment of the invention, sensors 60 of this type are provided thatcomprise a radiation capture and analysis device that is capable ofdetermining an intensity spectrum of captured radiation over a range ofwavelengths. In general, the temperature may be determined mostaccurately if a wide range of wavelengths are chosen. However, for thetemperatures of interest in the present application, it iscost-efficient to choose a limited wavelength range encompassing and/orcentered on the infra-red radiation band.

FIG. 20 shows an embodiment of the invention, in which local miniaturetemperature control systems 600 are built into the substrate table WT.In the example shown, these control systems 600 are positioned near thetips of raised portions of the substrate table WT (burl tops 640), whichare in turn in contact with the substrate W. Each miniature controlsystem 600 comprises a miniature temperature sensor 610, which may berealized as a micro-power integrated circuit temperature sensor, and aminiature heater 620, which may be realized as an integrated circuitheater (dissipating heat resistively). The miniature control systems 600are arranged so that the heater component 620 is activated to emit heatwhen the local temperature of the substrate, as measured by theminiature temperature sensor 610, falls below a predetermined thresholdvalue. Once the temperature is increased such that it passes againthrough the threshold value, the miniature control system 600 isconfigured to switch the miniature heater off. This arrangement may havethe advantages of being able to provide highly localized temperaturecontrol due to the miniature size of the control systems 600 and also ofnot needing a separate external control system to control the heaters620. Only two wires (connection 630) will be required to supply voltageto all of the miniature control systems 600 in the substrate table WT.Construction of the miniature temperature sensors 600 in the burl tops640 may be carried out by forming the substrate table WT from a siliconwafer. Microfabrication techniques, such as MEMS(Micro-Electro-Mechanical Systems), and CMOS (Complementary Metal OxideSemiconductor) technologies may be used to provide an exact reproductionof the standard substrate table WT construction, while also adding theintegrated-circuit temperature sensor/heater 610/620 on each burl top640 and providing the means for connecting them electrically to theexternal world (connection 630).

FIG. 21 depicts an embodiment of the invention comprising a projectionsystem controller 710, configured to adjust the properties of thepatterned radiation beam in response to measurements of the substrateand/or substrate table temperatures made by temperature sensors 60. Inthe embodiment shown, a plurality of temperature sensors 60 are embeddedin the substrate table WT. However, it would also be within the scope ofthe invention to provide temperature sensors elsewhere, such as in thebarrier member 12, and/or for only a single temperature sensor to beprovided.

As discussed above, the evaporation of immersion liquid on the substrateW may lead to substrate cooling, the resulting distortions possiblyleading to overlay errors, defocus, aberration, etc. According to thepresent embodiment, the projection system controller 710 is capable ofadjusting parameters of the patterned projection beam, such as itsoverall scaling, positional offset etc., so as to compensate for thethermally-induced distortions of the substrate W. As a simple example,if the projection system controller 710 receives input from thetemperature sensors 60 indicated that the substrate W is, to a firstapproximation, uniformly below a target temperature, it may beconfigured to scale the patterned projection beam by a factor so as toreduce the size of the image generated on the cooled substrate W. Wherethe temperature of the substrate W and/or substrate table WT is measuredby a plurality of temperature sensors 60, such that a temperatureprofile is obtained, more complex corrections may be implemented by theprojection system controller 710 in order to reduce errors such asoverlay errors, defocus and aberration. This approach may provide arapid way for responding to sudden changes in temperature without havingto incorporate heating elements in either the barrier member 12 or thesubstrate table WT, which may be expensive to implement and/or interferewith the dynamic performance of the substrate table WT. This form ofcompensation may have the added advantage of not being dependent on theparticular cooling mechanism at work and may be applicable to situationsin which at least a contribution to the change in temperature of thesubstrate W occurs due to processes other than evaporation of theimmersion liquid.

In the embodiment shown in FIG. 21, a thermally-induced-distortioncalculator 700 is also provided to translate the readings taken by thetemperature sensors 60 to an estimated distortion of the substrate W.This is achieved by first deriving a temperature profile of thesubstrate W and then using the known thermal properties of the substrateW, such as the thermal expansion coefficient of the substrate material,to calculate the thermally-induced distortion. To a first approximation,the relative distortion of a portion of the substrate W will beproportional to a temperature difference between that of the portion anda reference operating temperature (corresponding to zero relativedistortion). In the embodiment shown, the temperature sensors 60 areembedded in the substrate table WT so that additional calculations needto be carried out to derive the substrate temperature profile from thetemperature sensor readings. How this may be achieved is described belowin relation to this and other embodiments of the invention.

A measure of the temperature of the substrate W may be determinedaccording to several embodiments discussed above by temperature sensors60 positioned in the substrate table WT. This arrangement may haveconstructional advantages as there is relatively more space to positionthe sensors, they may be robustly and accurately positioned, and theymay be more easily serviced by whatever electrical connections arerequired. As discussed earlier, positioning sensors at a distance fromthe substrate W in the substrate table WT may also provide an effectiveway to sample a larger area of substrate W per sensor 60. However, itshould be understood that while the temperature of the materialimmediately surrounding the temperature sensor 60 may give anapproximate indication of the temperature of the substrate W, it ispossible to obtain a more accurate picture of the substrate temperatureprofile by further analysis, as described below. This analysis may beimplemented as part of any of the embodiments described above comprisingtemperature sensors 60 positioned in the substrate table WT.

Assuming that the heat transport from the substrate surface to the levelin the substrate table WT where the temperature sensors 60 arepositioned may be described as:

T _(∞) ^(chuck) −T _(current) ^(check) =f(ΔT ^(substrate)),   (1)

where T_(∞) ^(chuck) is the initial temperature of the substrate W,T_(current) ^(chuck) is the current temperature of a region of thesubstrate measured by a sensor 60 embedded in the substrate table WT,and ΔT^(substrate) is the temperature difference at substrate level forthe region in question, the temperature of the substrate region, andtherefore a temperature profile for the substrate as a whole (ifrequired), may be obtained based on this relationship. For example, thefollowing model may be used:

T _(∞) ^(chuck) −T _(current) ^(chuck) =k·e ^(−ΔT) ^(substrate) ^(/τ),  (2)

from which it follows that

ΔT ^(substrate) =−τln((T _(∞) ^(chuck) −T _(current) ^(chuck))/k),   (3)

which provides an expression for the temperature difference at substratelevel based only on parameters T and k, which in turn may be estimatedfrom test data.

Similar analyses may be used to derive a better measure of the substratetemperature from infra-red temperature sensor signals. Here, a problemis that silicon (which is frequently used as a substrate material) issignificantly transparent to infra-red, so that infra-red sensorspositioned in the barrier member 12 “looking down” on the substrate Wmay receive a mixture of radiation emitted from both the substrate W andthe substrate table WT immediately below it.

As mentioned above, as the barrier member 12 moves relative to thesubstrate W, a thin film of liquid may be left behind on the top surfaceof the substrate W in the wake of the barrier member 12. If no effectivecounter measures are taken, evaporation of this liquid may extract heatfrom the substrate W and/or substrate table WT. The resulting decreasein the temperature of the substrate W and/or substrate table WT may leadto shrinking which may in turn lead to overlay error, general loss ofperformance/resolution and/or yield loss of the device to bemanufactured. Several solutions to this issue have been described above,including providing a net of substrate heating channels and/or an arrayof independently controlled electrical heaters. However, it may bedifficult to coordinate the operation of these heating mechanisms insuch a way that heat is only generated where evaporation actuallyoccurs. Therefore, it may be difficult to ensure that temperaturegradients within the substrate W are reduced or minimized.

According to an embodiment of the invention, the lithographic apparatusis provided with a device to heat immersion liquid left in the wake ofthe barrier member 12 using microwave radiation. The frequency of themicrowave radiation can be tuned so as predominantly to heat theimmersion liquid directly, and not to couple to the surroundingapparatus elements (such as the substrate table WT, substrate W, barriermember 12, etc.). Heating power may therefore be directed precisely towhere it is required and temperature gradients may thereby be reduced orminimized. In principle, the heat needed to evaporate the liquid may becompletely provided by the microwave source so that no heat is extractedfrom the substrate W.

FIG. 22 shows an example arrangement, comprising a microwave source 800,configured to provide microwave radiation suitable to heat the immersionliquid being used, and a microwave containment cage 810, designed tocontain the microwave radiation within a region of interest (and toprotect regions, such as the immersion liquid reservoir 25, whereheating is not desirable). In the embodiment shown, the region ofinterest covers a substantially peripheral region of the substrate Wsurrounding the barrier member 12. The size of the region covered by themicrowave containment cage 810 may be chosen to be large enough that themicrowave radiation can completely evaporate liquid left behind in thewake of the barrier member 12 before the substrate table WT has movedfar enough relative to the barrier member 12 that the liquid would leavethe region exposed to the microwaves. The size of the microwavecontainment cage 810 should therefore be a function of the intensity ofthe microwave radiation to be maintained in the containment cage 810,the speed at which the substrate table WT moves under the barrier member12, and the quantity of liquid that is expected to be left behind in thewake of the barrier member 12.

The microwave containment cage 810 may be formed from a metallicmaterial, with openings of a size suitable to ensure substantiallycomplete reflection of the microwaves. Propagation of the microwaveradiation within the microwave containment cage 810 is shownschematically via arrows 830. The power of the microwave source 800,which determines the rate at which liquid left behind on the substrate Wwill be heated, may be selected on the basis of calibrationmeasurements. For example, test measurements may be carried out for anumber of different microwave source powers in order to determine whichpower leads to a minimum overlay error, for example. Alternatively, oneor more temperature sensors 60 may be provided and incorporated into afeedback loop controlled by a microwave source controller 820 via dataconnection 850, the microwave source controller 820 configured tocontrol the microwave source 800 via data connection 840. Thisarrangement may be advantageous where the speed of the substrate tableWT varies with time and/or when the amount of immersion liquid escapingfrom the barrier member 12 varies. The feedback mechanism may also beuseful when the microwave heating arrangement is to be used incombination with other temperature compensation methods, which may varyin efficiency with time. As in the feedback arrangements discussedabove, the feedback loop here may involve adjusting the power of themicrowave source 800 in order that the temperatures measured by thetemperature sensor(s) 60 converge towards one or more targettemperatures. It is envisaged that the microwave source 800 will emitmicrowave wavelength radiation. However, if the wavelength of radiationthat couples most effectively to the immersion liquid being used happensto be outside of the conventional range of wavelengths normallyassociated with microwaves, it is to be understood that the source 800could be adapted to emit the appropriate wavelengths of radiation (forexample, in the infrared or visible spectra).

FIG. 23 shows an arrangement of substrate heaters that can adjust thespatial variation in heat output according to the spatial variation insubstrate temperature without the need for a complex arrangement oftemperature sensors and/or external control systems. This is achieved byforming one or more electrically conductive strips 900 near the surfaceof the substrate table WT, in good thermal contact with the substrate W.For example, the electrically conductive strip(s) 900 may be formed bycoating an electrically conductive material on a top surface of thesubstrate table WT. In the embodiment shown, an external current source920 (not shown) is provided that passes a constant current 910 througheach of the one or more electrically conductive strips 900. According toa first variation, a single current source 920 may be provided thatpasses the same current through each of the one or more electricallyconductive strips 900. Alternatively, a plurality of current sources 920may be provided that are configured to pass different currents throughthe one or more electrically conductive strips 900. In either case, thecurrent that is passed through each electrically conductive strip shouldbe held constant so that the rate of heat produced by resistive heatingper unit length depends only on the local resistivity of the materialforming the electrically conductive strips 900. According to the presentembodiment, the material is chosen to have a negative temperaturedependence (i.e. so that an increase in temperature leads to a decreasein resistivity), desirably a strongly negative temperature dependence,so that significantly more heat is generated in cooler regions (whichhave higher resistivity) than in relatively warmer regions of each ofthe electrically conductive strips 900. In this way, more heating poweris naturally directed to those regions which need to be heated the most,thus reducing temperature gradients. In particular, the current 910 maybe varied until the difference in heating power between the coolerregions and the warmer regions is such as to substantially compensatethe heat extracted by evaporation of liquid on the surface of thesubstrate W (which, as discussed above, may be a major contributor tosubstrate temperature inhomogeneity). The heat generated in any givensegment of one of the electrically conductive strips is expected to beproportional to the square of the current multiplied by the resistanceof the segment.

In the above embodiment, the temperature dependent resistivity of theelectrically conductive strip(s) 900 was used directly to providetemperature dependent substrate heating by using the one or moreelectrically conductive strips themselves as heaters. According to analternative embodiment of the invention, the one or more electricallyconductive strips 900 can be used as one or more temperature sensors,possibly in combination with their function as one or more heatingelements. FIG. 24 shows an arrangement according to this embodiment.Again, a current 910 is passed through an electrically conductive strip900, which is arranged to have a temperature dependent resistivity. Inan embodiment, as before, the temperature dependent resistivity isstrongly negative, but a weaker temperature dependence and/or a positivetemperature dependence may still be tolerable. According to thisembodiment, one or more separate local substrate heaters 930 areprovided which are each powered by a localized power supply/amplifier950. The power supplied to each substrate heater 930 is controlled byreference to a measurement of the local resistivity in a segment of anelectrically conductive strip 900 that is closest to the substrateheater 930 in question. This may be achieved, for example, by measuringthe potential difference between the nearest pair of electrodes 940, asshown in FIG. 24. As before, the local resistivity of the electricallyconductive strip 900 is a function of the local temperature.

Calibration measurements may be used to establish the relationshipbetween the resistivity and the local temperature of the substrate W andthe power supply/amplifier 950 may be configured to adjust the power ofthe substrate heater 930 based on the difference between the measuredresistivity and the resistivity corresponding to a desired temperature.

The above arrangement may have an advantage of not being restricted bythe strength of the temperature dependence of the resistivity of theelectrically conductive strip(s) 900 and can, in principle, provide amore strongly spatially dependent heating power to the substrate W.Providing a large number of pairs of power supplies/amplifiers 950 andsubstrate heaters 930 allows a high spatial resolution. In addition,because the heating power to be provided to the substrate heater 930 isdetermined via a simple measurement of the resistivity of a nearbysegment of an electrically conductive strip 900, there is no need forcomplex and bulky electronics at the substrate table level, or forcomplex control electronics provided externally. The amplificationfactor (or amplification function: the function describing how the localheater power should vary as the temperature offset from the desiredtemperature varies) provided by the one or more powersupplies/amplifiers 950 may be determined beforehand by reference tocalibration measurements and may be provided in hardware.

FIG. 25 shows an embodiment of the invention in which the substrate W isheated by induction heaters. This approach may have an advantage thatheating is provided predominantly where it is required (e.g. in theproximity of the barrier member 12). An induction source 960 acts toprovide the induction heating power by coupling to an induction element970 formed within the substrate table WT and, in an embodiment, in aposition so as to be in good thermal contact with the substrate W. Thepower output of the induction source 960 is controlled in turn by aninduction controller 980. The induction controller 980 may vary thepower of the induction source 960 according to a pre-set routine (forexample, so as to heat predominantly the region of the substrate W whichrecently passed the barrier member 12). A plurality of spatiallyseparated induction sources and/or elements may be provided, eachconfigurable, for example, to provide the same or a different heatingpower, in order that heating may be provided to the substrate W in sucha way that temperature gradients may be reduced. Alternatively, theinduction controller 980 may use a feedback model. This may beconfigured to vary the output power of the one or more induction sources960 so that the temperature measured by one or more temperature sensors60 (connected to the induction controller 980) converge on one or morecorresponding target temperatures. The use of induction heating to heatthe substrate table WT may have a further advantage in that only minoralterations may need to be made to the substrate table WT (such asadding the induction element 970). Therefore, the mechanical operationof the substrate table WT may not be significantly interfered with. Thefact that the induction source 960 is mechanically separated from theinduction element 970 is advantageous also from the point of view ofupgradeability: each component may be adapted to a large extentindependently of the other.

As described above, one approach to reducing cooling caused by immersionliquid evaporating from the surface of the substrate W, is to supplyhumidified gas to the gas seal 27 (“humidified gas” being understood ina broad sense to mean gas that contains a significant proportion of theimmersion liquid in vapor form). Where the atmosphere above thesubstrate W contains a higher proportion of immersion liquid vapor, thedynamic equilibrium that is established between evaporation of theliquid from the substrate W and condensation of the liquid vapor ontothe substrate W is such that the net evaporation rate is lower than ifthe atmosphere above the substrate W was dry (i.e. not containing anysignificant quantity of immersion liquid vapor). In order for thismechanism to work in a reproducible and consistent manner, a reliablemechanism to humidify the gas supplied to the gas seal 27 should beprovided. According to an embodiment, which is illustrated schematicallyin FIG. 26, a humidifying section 1000 is provided for this purpose. Gasis input to the humidifying section 1000 from a clean gas source (forexample) via conduit 1005 to evaporation vessel 1010. The evaporationvessel 1010 comprises one or more internal heating elements that heatone or more baths of immersion liquid in order to produce immersionliquid vapor. The immersion liquid vapor thus produced is mixed with theclean gas supplied via conduit 1005 and output from the evaporationvessel 1010 via conduit 1015. The partially saturated gas is then inputto condensation vessel 1020 (also referred to as a “cooling vessel”), inwhich it is cooled to such an extent that the mixture of gas andimmersion liquid vapor becomes over saturated and immersion liquidcondenses out of the mixture. What remains in gaseous form is very closeto or exactly 100% saturated with immersion liquid vapor at the workingtemperature of the condensation vessel 1020. This 100% saturated gassupply is then input via conduit 1025 to mixing chamber 1040 where itmay be mixed with a source of dry gas input at a controlled rate viaconduit 1035 from dry gas source 1030 in order to enable the output ofgas, via conduit 1045, at a controlled temperature and/or controlledlevel of saturation, which may then be supplied to the gas seal 27, forexample.

An alternative system for humidifying gas is to pass it through aso-called bubbler, which is a porous device immersed in a vesselcontaining liquid and liquid vapor. The gas becomes increasinglysaturated with liquid as it passes through. However, it may be difficultin this arrangement to control the level of saturation or humidity ofthe gas that is produced. Variations in flow, in vessel temperature, orin liquid level may all influence the amount of liquid vapor held in thegas that leaves the system. In particular, it may be difficult toachieve 100% saturation using this method. Optimizing the performance ofsuch a system may require the design of a relatively complex apparatus,for example to ensure adequate and reproducible contact between theliquid and gas.

As described above, evaporation of immersion liquid from the substrate Wmay have negative effects on the performance of the lithographicapparatus. Contamination in the liquid may cause particle contamination(also known as water stains) on the substrate W. Evaporation may alsonegatively effect overlay performance, focus and optical performancebecause of cooling effects. Humidified gas in the gas seal 27 may beused to minimize evaporation. According to one approach, 100% saturatedgas is desirable so as to achieve zero net evaporation from the surfaceof the substrate W. An embodiment has been described above that isdesigned to produce 100% saturated gas in a controlled manner. However,the relative humidity of the gas inevitably drops as it is output fromthe gas seal 27 due to expansion of the gas. In practice, this may meanthat at the point of use (i.e. at the substrate surface) the maximumachievable humidity may be substantially lower than 100%, for examplearound 60%, when the gas is delivered at the working temperature of thesubstrate W (for example 22° C.). If an atmosphere of less than 100%relative humidity is maintained above the substrate surface, some netevaporation will occur.

According to an embodiment, which is illustrated schematically in FIG.27, the humidity of the gas after it has left the gas seal 27 and hasexpanded, is controlled (increased) by increasing the temperature of thegas supplied to the gas seal.

In this scenario, the hot gas leaving the gas seal 27 is suddenlyexposed to an environment at a lower temperature (i.e. the normalworking temperature of the lithographic apparatus) and cools. Coolingtends to increase the level of saturation or relative humidity. Theoverall temperature drop may be controlled to compensate closely for theexpansion of the gas and the associated reduction in saturation.

For a typical system maintained at a working temperature of 22° C. withgas supplied to the gas seal 27 close to saturation (for example 90-100%relative humidity), and a typical barrier member pressure drop of 0.4bars, a temperature offset of between 1 and 5 K may be adequate formaintaining near 100% relative humidity in the gas left above thesubstrate W outside of the gas seal 27. Careful design of the system maybe required to prevent condensation of the highly saturated gas beforeit leaves the gas seal 27. For example, the walls of the conduit leadingto the gas seal 27 through the barrier member 12 should be thermallyinsulated so as to isolate the hot gas from the cool barrier member 12and prevent condensation on the walls of the conduit.

FIG. 27, mentioned above, shows an example arrangement for controllingthe temperature of the gas to be supplied to the gas seal 27, which maybe positioned, for example, between the humidifying section 1000 and thegas seal 27. Relatively cool saturated gas is supplied by conduit 1045to heat exchanger 1100, which heats the saturated gas to a targettemperature by exchanging heat with a heat exchange fluid provided byheater 1110. The heater 1110 provides heat exchange fluid via outputline 1120 at temperature T1 and receives heat exchange fluid via inputline 1130 at temperature T2, where T1 is greater than T2. The heater1110 may heat the heat exchange fluid by means of a Peltier heater, forexample. According to an example arrangement, a Peltier heater isprovided that works in the 500 watt to 1500 watt range to generatetemperature controlled liquid (e.g., water) to an accuracy of ±0.01° C.with a set point of 27° C.

According to an embodiment of the invention illustrated schematically inFIG. 28, a highly purified stream of humidified gas is created using ahumidifier cabinet 1200, in which several evaporating units 1220 inparallel act to evaporate liquid. The temperature of the humidified gasproduced is controlled by supplying to each of the evaporators 1220 astream of temperature controlled heat exchange fluid by conduit 1205.The heat exchange fluid may be provided by heat exchange fluid source1110, which may also be used to control the temperature of the saturatedgas just before it is fed to the gas seal 27 as described above.Alternatively, a separate heat exchange fluid source may be provided.The humidified and temperature controlled gas is passed via hydrophobicfilters 1210 to output valve 1250 before being passed via heat exchanger1100 to the gas seal 27.

Changing or even stopping the humidified gas flow from the humidifiercabinet 1200 changes the equilibrium and may require a longstabilization time before humidified gas may again be supplied to thegas seal 27 with a well controlled temperature and degree of saturation.However, due to the dynamic nature of the task performed by the barriermember 12 and gas seal 27, the rate of gas required by the gas seal 27may vary considerably with time: for example, there may be short periodswhere the gas seal 27 does not function. Rather than adapt the system sothat it can stabilize more quickly, which might require substantial andcomplex additional apparatus, the present embodiment includes a variablevent system 1240 which allows gas to be vented at a controlled rate toan external reservoir or to an exhaust. The vent system 1240 can beconfigured such that the flow from the humidifier cabinet 1200 remainsconstant. This may be achieved in practice by ensuring that the totalflow through main valve 1250 and vent system 1240 is constant. This maybe implemented by configuring the vent system 1240 to have a flowimpedance that is responsive to the reading of pressure gauge 1230,which corresponds to the backpressure “felt” by the humidifier cabinet1200. In particular, this pressure should be kept constant. Thisarrangement provides not only for better stability but also higherthroughput because the need for a stabilization time between differentoperational phases of the gas seal 27 may be avoided.

All of the above features can be combined in any combination and appliedwhere relevant to all types of liquid supply system including thosementioned in the background section above.

In an embodiment, there is provided a lithographic apparatus,comprising: a substrate table constructed to hold a substrate; aprojection system configured to project a patterned radiation beam ontoa target portion of the substrate; a liquid supply system configured toat least partly fill a space between the projection system and thesubstrate with liquid; a barrier member arranged to substantiallycontain the liquid within the space between the projection system andthe substrate; and a liquid evaporation controller arranged to control anet rate of evaporation of liquid supplied by the liquid supply system.

In an embodiment, the apparatus further comprises a gas seal connectedto a gas source, configured to control the amount of liquid that escapesfrom the barrier member through a gap delimited on one side by aboundary of the barrier member and on a second side by the substrate,wherein the liquid evaporation controller comprises a gas humiditycontroller configured to interact with the gas source to provide gaswith a controlled relative humidity of greater than about 10%. In anembodiment, the gas humidity controller is configured to produce astream of humidity-controlled gas at a constant flow rate and comprisesa gas seal flow-rate controller configured to receive the constantstream from the humidity controller and vary the rate of flow of gas tothe gas seal by selectively venting a portion of the constant streamsupplied by the humidity controller to an external reservoir. In anembodiment, the humidity controller comprises a humidifying sectionconfigured to humidify a stream of gas to a controlled extent, thehumidifying section comprising: an evaporation vessel configured toreceive a stream of relatively dry gas and at least partially humidifythe stream of gas with liquid vapor evaporated from a bath; and acooling vessel configured to be held at a temperature substantiallylower than that of the evaporation vessel and to receive and cool the atleast partially humidified gas stream in order to obtain a stream ofcompletely saturated gas. In an embodiment, a saturated gas output ofthe cooling vessel is connectable to a dry gas source, and wherein thegas humidity controller is configured to adjust the rate at which drygas is mixed with the stream of saturated gas output from the coolingvessel in order to obtain a stream of gas with a controlled relativehumidity level. In an embodiment, the apparatus further comprises a gastemperature controller arranged to interact with the gas source tocontrol the temperature of the gas supplied to the gas seal, wherein thetemperature of the gas before entering the gas seal is arranged to behigher than the average temperature of the substrate. In an embodiment,the temperature of the gas before entering the gas seal is arranged tobe between 1 and 5 K higher than the average temperature of thesubstrate. In an embodiment, the temperature of the humidified gassupplied to the gas seal is such as to achieve a desired level ofhumidity after expansion of the gas in the gas seal. In an embodiment,the gas humidity controller is configured to provide gas with a relativehumidity greater than 40%. In an embodiment, the apparatus furthercomprises a temperature sensor configured to measure the temperature ofat least a portion of the substrate, or at least a portion of thesubstrate table, or at least a portion of a substrate holder, or anycombination thereof, and wherein the humidity controller is capable ofadjusting the relative humidity of the gas supplied by the gas source soas to reduce a difference between the temperature measured by thetemperature sensor and a target temperature. In an embodiment, theliquid evaporation controller comprises a gas-shower outlet configuredto supply gas with a controlled relative humidity of greater than about10% to the region outside of the barrier member between the substrateand the projection system. In an embodiment, the gas-shower outlet isconfigured to supply gas with a relative humidity in the range of about40% to about 50%. In an embodiment, the apparatus further comprises: atemperature sensor configured to measure the temperature of at least aportion of the substrate, or at least a portion of the substrate table,or at least a portion of a substrate holder, or any combination thereof;and a gas-shower outlet controller capable of adjusting the relativehumidity of the gas supplied by the gas-shower outlet so as to reduce adifference between the temperature measured by the temperature sensorand a target temperature. In an embodiment, the apparatus furthercomprises: a gas seal connected to a gas source, configured to controlthe amount of liquid that escapes from the barrier member through a gap,delimited on one side by a boundary of the barrier member and on asecond side by the substrate, wherein the gas source provides gas with acontrolled relative humidity of greater than 10%; and a gas-showeroutlet configured to supply gas with a controlled relative humidity,substantially equal to that of the gas supplied by the gas source, tothe region outside of the barrier member between the substrate and theprojection system.

In an embodiment, there is provided a lithographic apparatus,comprising: a substrate table constructed to hold a substrate; aprojection system configured to project a patterned radiation beam ontoa target portion of the substrate; a liquid supply system configured toat least partly fill a space between the projection system and thesubstrate with liquid; a barrier member arranged to substantiallycontain the liquid within the space between the projection system andthe substrate; a substrate table displacement system, arranged to movethe substrate table along a predetermined path relative to the barriermember, thereby moving the target portion over the surface of thesubstrate; and a microwave source and microwave containment device,together configured to supply heat to liquid on the surface of thesubstrate.

In an embodiment, the microwave containment device comprises a metalliccage and defines a volume within which microwave radiation produced bythe microwave source can propagate. In an embodiment, the microwavecontainment device is fixed relative to the barrier member and thevolume extends to cover only a sub-region of the surface of thesubstrate around the barrier member, at any one time.

In an embodiment, there is provided a lithographic apparatus,comprising: a substrate table constructed to hold a substrate; aprojection system configured to project a patterned radiation beam ontoa target portion of the substrate; a liquid supply system configured toat least partly fill a space between the projection system and thesubstrate with liquid; a barrier member arranged to substantiallycontain the liquid within the space between the projection system andthe substrate; a substrate table displacement system, arranged to movethe substrate table along a predetermined path relative to the barriermember, thereby moving the target portion over the surface of thesubstrate; and a substrate heater configured to heat at least a portionof the substrate according to a position of the substrate table relativeto the barrier member, or a velocity of the substrate table relative tothe barrier member, or an acceleration of the substrate table relativeto the barrier member, or a predetermined path of the substrate tablerelative to the barrier member, or a local substrate temperature, or alocal substrate table temperature, or any combination thereof.

In an embodiment, the substrate heater comprises an induction heater, ora visible-light source, or an infrared emission source, or a glow wireelectrical resistance heater, or a temperature-controlled gas jet, orany combination thereof. In an embodiment, the induction heater isconfigured to heat the substrate via an induction plate associated withthe substrate table and formed of a material suitable for inductionheating. In an embodiment, the substrate heater comprises a plurality oflocal heaters, each capable of heating substantially separate portionsof the substrate, wherein the local heaters are arranged to be switchedto a heat emitting state when positioned to heat a region of thesubstrate over which the barrier member has already passed, and to beswitched to a non-heat emitting state when positioned to heat a regionof the substrate over which the barrier member has yet to pass. In anembodiment, the apparatus further comprises a substrate table pathdetermining device, configured to determine the position of thesubstrate table, or the velocity of the substrate table, or theacceleration of the substrate table, or the predetermined path of thesubstrate table, or any combination thereof. In an embodiment, thesubstrate heater comprises a plurality of remote heaters positionedaround the periphery of the barrier member. In an embodiment, the poweroutput of the remote heaters is controlled depending on the direction ofmotion of the substrate table relative to the barrier member, asdetermined by a substrate table path determining device. In anembodiment, remote heaters at positions closest to a leading edge of thebarrier member are configured to provide a lower power output thanremote heaters positioned closest to a trailing edge of the barriermember. In an embodiment, the apparatus further comprises: a temperaturesensor configured to measure the temperature of at least a portion ofthe substrate, or of at least a portion of the substrate table, or of atleast a portion of a substrate holder, or any combination thereof; and asubstrate temperature controller, arranged to control the output of eachof the plurality of remote heaters so as to reduce a difference betweenthe temperature measured by the temperature sensor and a targettemperature. In an embodiment, the temperature sensor comprises aradiation capture and analysis device capable of determining anintensity spectrum of captured radiation over a range of wavelengthsincluding infrared. In an embodiment, the substrate heater comprises alocal heater arranged to heat predominantly a distinct portion of thesubstrate and the apparatus further comprises: a temperature sensorconfigured to measure a temperature of at least a portion of thesubstrate, or of at least a portion of the substrate table, or of atleast a portion of a substrate holder, or any combination thereof; and asubstrate temperature controller, arranged to control the output of thelocal heater so as to reduce a difference between the temperaturemeasured by the temperature sensor and a target temperature. In anembodiment, the local heater is arranged to follow substantially apredetermined path of the substrate table relative to the barriermember. In an embodiment, the substrate heater comprises a liquidtemperature controller arranged to interact with the liquid supplysystem to control the temperature of the liquid to be filled in thespace between the projection system to a temperature greater than about295 K. In an embodiment, the apparatus further comprises a gas sealconfigured to control the amount of liquid that escapes from the barriermember through a gap delimited on one side by a boundary of the barriermember and on a second side by the substrate, the gas seal beingsupplied with pressurized gas by a pressurized gas supply system, andwherein the substrate heater comprises a gas temperature controllerarranged to interact with the pressurized gas supply system to controlthe temperature of the pressurized gas supplied to the gas seal to atemperature greater than about 300 K. In an embodiment, the substrateheater comprises elongated elements arranged in substantially parallelstrips, the strips being oriented substantially perpendicular to ascanning direction of the substrate table relative to the barriermember, the separation of the strips being arranged to increaseprogressively from a first strip, corresponding to a region of asubstrate in which the projection system is configured to project theradiation beam during a first time period, to a final strip,corresponding to a region of the same substrate in which the projectionsystem is configured to project the radiation beam during a time periodlater than the first time period. In an embodiment, each substantiallyparallel strip provides a uniform power per unit length along itslength. In an embodiment, the substrate heater comprises elongatedelements arranged in substantially parallel strips, the strips beingoriented substantially perpendicular to a scanning direction of thesubstrate table relative to the barrier member, and wherein the stripsare arranged to provide a progressively decreasing power per unit lengthof strip from a first strip, corresponding to a region of a substrate inwhich the projection system is configured to project the radiation beamduring a first time period, to a final strip, corresponding to a regionof the same substrate in which the projection system is configured toproject the radiation beam during a time period later than the firsttime period. In an embodiment, the substrate heater is arranged toprovide a higher heating power at target regions on a substrate wherethe projection system is configured to project the patterned radiationbeam at a first time and progressively lower heating powers at targetregions on the same substrate where the projection system is configuredto project the patterned radiation beam at later times. In anembodiment, the substrate heater comprises an array of individuallyaddressable local heaters and a heater array controller adapted tocontrol actuation of the individually addressable local heatersaccording to a predetermined algorithm, the predetermined algorithmcontrolling actuation with respect to heater position, or timing, oramount of heat generated, or a rate of heat generated, or anycombination thereof. In an embodiment, the substrate heater comprises:an electrically conductive strip arranged to be in good thermal contactwith a portion of the substrate, the resistivity of the electricallyconductive strip selected so that resistive heating from electricalcurrent is higher in relatively cooler regions of the substrate than itis in relatively warmer regions of the substrate; and a current sourceconfigured to pass a current of controlled magnitude through theelectrically conductive strip. In an embodiment, the magnitude of thecurrent is such as to minimize a temperature gradient caused byevaporation of liquid from the surface of the substrate. In anembodiment, the conductive strip is formed from a material with anelectrical resistance that decreases with increasing temperature. In anembodiment, the apparatus further comprises a plurality of localsubstrate heaters, each configured to supply heat to a localizedsub-region of the substrate; a local substrate heater controllerconfigured to control the output power of at least a subset of theplurality of local substrate heaters; and a plurality of resistivitymeasuring devices, each configured to measure the resistivity of atleast a portion of the electrically conductive strip, wherein the outputpower of each of the local substrate heaters is controlled according tothe resistivity of a portion of the electrically conductive stripnearest to the localized sub-region to be heated and measured by one ofthe resistivity measuring devices. In an embodiment, the apparatusfurther comprises a constant current source configured to pass a currentof controlled magnitude through the electrically conductive strip andwherein one of the resistivity measuring devices operates by measuring apotential difference arising between at least two points on theelectrically conductive strip.

In an embodiment, there is provided a device manufacturing method,comprising: projecting a patterned radiation beam through a liquid ontoa target portion of a substrate; and controlling a net rate ofevaporation of the liquid.

In an embodiment, there is provided a device manufacturing method,comprising: projecting a patterned radiation beam through a liquid ontoa target portion of the substrate; moving a substrate table holding thesubstrate along a predetermined path relative to a seal member used tocontain the liquid, thereby moving the target portion over a surface ofthe substrate; and heating liquid on the surface of the substrate usingmicrowaves.

In an embodiment, there is provided a device manufacturing method,comprising: projecting a patterned radiation beam through a liquid ontoa target portion of a substrate; moving a substrate table holding thesubstrate along a predetermined path relative to a seal member used tocontain the liquid, thereby moving the target portion over a surface ofthe substrate; and heating at least a portion of the substrate accordingto a position of the substrate table relative to the seal member, or avelocity of the substrate table relative to the seal member, or anacceleration of the substrate table relative to the seal member, or apredetermined path of the substrate table relative to the seal member,or a local substrate temperature, or a local substrate tabletemperature, or any combination thereof.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,flat-panel displays, liquid-crystal displays (LCDs), thin-film magneticheads, etc. The skilled artisan will appreciate that, in the context ofsuch alternative applications, any use of the terms “wafer” or “die”herein may be considered as synonymous with the more general terms“substrate” or “target portion”, respectively. The substrate referred toherein may be processed, before or after exposure, in for example atrack (a tool that typically applies a layer of resist to a substrateand develops the exposed resist), a metrology tool and/or an inspectiontool. Where applicable, the disclosure herein may be applied to such andother substrate processing tools. Further, the substrate may beprocessed more than once, for example in order to create a multi-layerIC, so that the term substrate used herein may also refer to a substratethat already contains multiple processed layers.

Although specific reference may have been made above to the use ofembodiments of the invention in the context of optical lithography, itwill be appreciated that the invention may be used in otherapplications, for example imprint lithography, and where the contextallows, is not limited to optical lithography. In imprint lithography atopography in a patterning device defines the pattern created on asubstrate. The topography of the patterning device may be pressed into alayer of resist supplied to the substrate whereupon the resist is curedby applying electromagnetic radiation, heat, pressure or a combinationthereof. The patterning device is moved out of the resist leaving apattern in it after the resist is cured.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength of or about 365, 248, 193, 157 or 126 nm) andextreme ultra-violet (EUV) radiation (e.g. having a wavelength in therange of 5-20 nm), as well as particle beams, such as ion beams orelectron beams.

The term “lens”, where the context allows, may refer to any one orcombination of various types of optical components, includingrefractive, reflective, magnetic, electromagnetic and electrostaticoptical components.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. For example, the invention may take the form of acomputer program containing one or more sequences of machine-readableinstructions describing a method as disclosed above, or a data storagemedium (e.g. semiconductor memory, magnetic or optical disk) having sucha computer program stored therein.

One or more embodiments of the invention may be applied to any immersionlithography apparatus, in particular, but not exclusively, those typesmentioned above and whether the immersion liquid is provided in the formof a bath or only on a localized surface area of the substrate. A liquidsupply system as contemplated herein should be broadly construed. Incertain embodiments, it may be a mechanism or combination of structuresthat provides a liquid to a space between the projection system and thesubstrate and/or substrate table. It may comprise a combination of oneor more structures, one or more liquid inlets, one or more gas inlets,one or more gas outlets, and/or one or more liquid outlets that provideliquid to the space. In an embodiment, a surface of the space may be aportion of the substrate and/or substrate table, or a surface of thespace may completely cover a surface of the substrate and/or substratetable, or the space may envelop the substrate and/or substrate table.The liquid supply system may optionally further include one or moreelements to control the position, quantity, quality, shape, flow rate orany other features of the liquid.

The descriptions above are intended to be illustrative, not limiting.Thus, it will be apparent to one skilled in the art that modificationsmay be made to the invention as described without departing from thescope of the claims set out below.

1. A lithographic apparatus, comprising: a substrate table constructedto hold a substrate; a projection system configured to project apatterned radiation beam onto a target portion of the substrate; aliquid supply system configured to at least partly fill a space betweenthe projection system and the substrate with liquid; a barrier memberarranged to substantially contain the liquid within the space betweenthe projection system and the substrate; and a liquid evaporationcontroller arranged to control a net rate of evaporation of liquidsupplied by the liquid supply system.
 2. The apparatus according toclaim 1, further comprising a gas seal connected to a gas source,configured to control the amount of liquid that escapes from the barriermember through a gap delimited on one side by a boundary of the barriermember and on a second side by the substrate, wherein the liquidevaporation controller comprises a gas humidity controller configured tointeract with the gas source to provide gas with a controlled relativehumidity of greater than about 10%.
 3. The apparatus according to claim2, wherein the gas humidity controller is configured to produce a streamof humidity-controlled gas at a constant flow rate and comprises a gasseal flow-rate controller configured to receive the constant stream fromthe humidity controller and vary the rate of flow of gas to the gas sealby selectively venting a portion of the constant stream supplied by thehumidity controller to an external reservoir.
 4. The apparatus accordingto claim 2, wherein the humidity controller comprises a humidifyingsection configured to humidify a stream of gas to a controlled extent,the humidifying section comprising: an evaporation vessel configured toreceive a stream of relatively dry gas and at least partially humidifythe stream of gas with liquid vapor evaporated from a bath; and acooling vessel configured to be held at a temperature substantiallylower than that of the evaporation vessel and to receive and cool the atleast partially humidified gas stream in order to obtain a stream ofcompletely saturated gas, wherein a saturated gas output of the coolingvessel is connectable to a dry gas source, and wherein the gas humiditycontroller is configured to adjust the rate at which dry gas is mixedwith the stream of saturated gas output from the cooling vessel in orderto obtain a stream of gas with a controlled relative humidity level. 5.(canceled)
 6. The apparatus according to claim 2, further comprising agas temperature controller arranged to interact with the gas source tocontrol the temperature of the gas supplied to the gas seal, wherein thetemperature of the gas before entering the gas seal is arranged to behigher than the average temperature of the substrate, and wherein thetemperature of the gas before entering the gas seal is arranged to bebetween 1 and 5 K higher than the average temperature of the substrateor the temperature of the humidified gas supplied to the gas seal issuch as to achieve a desired level of humidity after expansion of thegas in the gas seal. 7-8. (canceled)
 9. The apparatus according to claim2, wherein the gas humidity controller is configured to provide gas witha relative humidity greater than 40%, or further comprising atemperature sensor configured to measure the temperature of at least aportion of the substrate, or at least a portion of the substrate table,or at least a portion of a substrate holder, or any combination thereof,and wherein the humidity controller is capable of adjusting the relativehumidity of the gas supplied by the gas source so as to reduce adifference between the temperature measured by the temperature sensorand a target temperature.
 10. (canceled)
 11. The apparatus according toclaim 1, wherein the liquid evaporation controller comprises agas-shower outlet configured to supply gas with a controlled relativehumidity of greater than about 10% to the region outside of the barriermember between the substrate and the projection system, wherein thegas-shower outlet is configured to supply gas with a relative humidityin the range of about 40% to about 50%, or further comprising: atemperature sensor configured to measure the temperature of at least aportion of the substrate, or at least a portion of the substrate table,or at least a portion of a substrate holder, or any combination thereof;and a gas-shower outlet controller capable of adjusting the relativehumidity of the gas supplied by the gas-shower outlet so as to reduce adifference between the temperature measured by the temperature sensorand a target temperature. 12-13. (canceled)
 14. The apparatus accordingto claim 1, further comprising: a gas seal connected to a gas source,configured to control the amount of liquid that escapes from the barriermember through a gap, delimited on one side by a boundary of the barriermember and on a second side by the substrate, wherein the gas sourceprovides gas with a controlled relative humidity of greater than 10%;and a gas-shower outlet configured to supply gas with a controlledrelative humidity, substantially equal to that of the gas supplied bythe gas source, to the region outside of the barrier member between thesubstrate and the projection system.
 15. A lithographic apparatus,comprising: a substrate table constructed to hold a substrate; aprojection system configured to project a patterned radiation beam ontoa target portion of the substrate; a liquid supply system configured toat least partly fill a space between the projection system and thesubstrate with liquid; a barrier member arranged to substantiallycontain the liquid within the space between the projection system andthe substrate; a substrate table displacement system, arranged to movethe substrate table along a predetermined path relative to the barriermember, thereby moving the target portion over the surface of thesubstrate; and a microwave source and microwave containment device,together configured to supply heat to liquid on the surface of thesubstrate.
 16. The apparatus according to claim 15, wherein themicrowave containment device comprises a metallic cage and defines avolume within which microwave radiation produced by the microwave sourcecan propagate or wherein the microwave containment device is fixedrelative to the barrier member and the volume extends to cover only asub-region of the surface of the substrate around the barrier member, atany one time .
 17. (canceled)
 18. A lithographic apparatus, comprising:a substrate table constructed to hold a substrate; a projection systemconfigured to project a patterned radiation beam onto a target portionof the substrate; a liquid supply system configured to at least partlyfill a space between the projection system and the substrate withliquid; a barrier member arranged to substantially contain the liquidwithin the space between the projection system and the substrate; asubstrate table displacement system, arranged to move the substratetable along a predetermined path relative to the barrier member, therebymoving the target portion over the surface of the substrate; and asubstrate heater configured to heat at least a portion of the substrateaccording to a position of the substrate table relative to the barriermember, or a velocity of the substrate table relative to the barriermember, or an acceleration of the substrate table relative to thebarrier member, or a predetermined path of the substrate table relativeto the barrier member, or a local substrate temperature, or a localsubstrate table temperature, or any combination thereof.
 19. Theapparatus according to claim 18, wherein the substrate heater comprisesan induction heater configured to heat the substrate via an inductionplate associated with the substrate table and formed of a materialsuitable for induction heating, or a visible-light source, or aninfrared emission source, or a glow wire electrical resistance heater,or a temperature-controlled gas jet, or any combination thereof. 20.(canceled)
 21. The apparatus according to claim 18, wherein thesubstrate heater comprises a plurality of local heaters, each capable ofheating substantially separate portions of the substrate, wherein thelocal heaters are arranged to be switched to a heat emitting state whenpositioned to heat a region of the substrate over which the barriermember has already passed, and to be switched to a non-heat emittingstate when positioned to heat a region of the substrate over which thebarrier member has yet to pass, or further comprising a substrate tablepath determining device, configured to determine the position of thesubstrate table, or the velocity of the substrate table, or theacceleration of the substrate table, or the predetermined path of thesubstrate table, or any combination thereof, or wherein the substrateheater comprises a liquid temperature controller arranged to interactwith the liquid supply system to control the temperature of the liquidto be filled in the space between the projection system to a temperaturegreater than about 295 K, or wherein the substrate heater is arranged toprovide a higher heating power at target regions on a substrate wherethe projection system is configured to project the patterned radiationbeam at a first time and progressively lower heating powers at targetregions on the same substrate where the projection system is configuredto project the patterned radiation beam at later times, or wherein thesubstrate heater comprises an array of individually addressable localheaters and a heater array controller adapted to control actuation ofthe individually addressable local heaters according to a predeterminedalgorithm, the predetermined algorithm controlling actuation withrespect to heater position, or timing, or amount of heat generated, or arate of heat generated, or any combination thereof.
 22. (canceled) 23.The apparatus according to claim 18, wherein the substrate heatercomprises a plurality of remote heaters positioned around the peripheryof the barrier member.
 24. The apparatus according to claim 23, whereinthe power output of the remote heaters is controlled depending on thedirection of motion of the substrate table relative to the barriermember, as determined by a substrate table path determining device. 25.The apparatus according to claim 24, wherein remote heaters at positionsclosest to a leading edge of the barrier member are configured toprovide a lower power output than remote heaters positioned closest to atrailing edge of the barrier member.
 26. The apparatus according toclaim 23, further comprising: a temperature sensor configured to measurethe temperature of at least a portion of the substrate, or of at least aportion of the substrate table, or of at least a portion of a substrateholder, or any combination thereof, wherein the temperature sensorcomprises a radiation capture and analysis device capable of determiningan intensity spectrum of captured radiation over a range of wavelengthsincluding infrared; and a substrate temperature controller, arranged tocontrol the output of each of the plurality of remote heaters so as toreduce a difference between the temperature measured by the temperaturesensor and a target temperature.
 27. (canceled)
 28. The apparatusaccording to claim 18, wherein the substrate heater comprises a localheater arranged to heat predominantly a distinct portion of thesubstrate and the apparatus further comprising: a temperature sensorconfigured to measure a temperature of at least a portion of thesubstrate, or of at least a portion of the substrate table, or of atleast a portion of a substrate holder, or any combination thereof; and asubstrate temperature controller, arranged to control the output of thelocal heater so as to reduce a difference between the temperaturemeasured by the temperature sensor and a target temperature, wherein thelocal heater is arranged to follow substantially a predetermined path ofthe substrate table relative to the barrier member. 29-30. (canceled)31. The apparatus according to claim 18, further comprising a gas sealconfigured to control the amount of liquid that escapes from the barriermember through a gap delimited on one side by a boundary of the barriermember and on a second side by the substrate, the gas seal beingsupplied with pressurized gas by a pressurized gas supply system, andwherein the substrate heater comprises a gas temperature controllerarranged to interact with the pressurized gas supply system to controlthe temperature of the pressurized gas supplied to the gas seal to atemperature greater than about 300 K, wherein the substrate heatercomprises elongated elements arranged in substantially parallel strips,the strips being oriented substantially perpendicular to a scanningdirection of the substrate table relative to the barrier member, theseparation of the strips being arranged to increase progressively from afirst strip, corresponding to a region of a substrate in which theprojection system is configured to project the radiation beam during afirst time period, to a final strip, corresponding to a region of thesame substrate in which the projection system is configured to projectthe radiation beam during a time period later than the first timeperiod, wherein each substantially parallel strip provides a uniformpower per unit length along its length, or wherein the substrate heatercomprises elongated elements arranged in substantially parallel strips,the strips being oriented substantially perpendicular to a scanningdirection of the substrate table relative to the barrier member, andwherein the strips are arranged to provide a progressively decreasingpower per unit length of strip from a first strip, corresponding to aregion of a substrate in which the projection system is configured toproject the radiation beam during a first time period, to a final strip,corresponding to a region of the same substrate in which the projectionsystem is configured to project the radiation beam during a time periodlater than the first time period. 32-36. (canceled)
 37. The apparatusaccording to claim 18, wherein the substrate heater comprises: anelectrically conductive strip arranged to be in good thermal contactwith a portion of the substrate, the resistivity of the electricallyconductive strip selected so that resistive heating from electricalcurrent is higher in relatively cooler regions of the substrate than itis in relatively warmer regions of the substrate; and a current sourceconfigured to pass a current of controlled magnitude through theelectrically conductive strip, wherein the magnitude of the current issuch as to minimize a temperature gradient caused by evaporation ofliquid from the surface of the substrate, or wherein the conductivestrip is formed from a material with an electrical resistance thatdecreases with increasing temperature, or further comprising: aplurality of local substrate heaters, each configured to supply heat toa localized sub-region of the substrate; a local substrate heatercontroller configured to control the output power of at least a subsetof the plurality of local substrate heaters; and a plurality ofresistivity measuring devices, each configured to measure theresistivity of at least a portion of the electrically conductive strip,a constant current source configured to pass a current of controlledmagnitude through the electrically conductive strip, wherein the outputpower of each of the local substrate heaters is controlled according tothe resistivity of a portion of the electrically conductive stripnearest to the localized sub-region to be heated and measured by one ofthe resistivity measuring devices, and wherein one of the resistivitymeasuring devices operates by measuring a potential difference arisingbetween at least two points on the electrically conductive strip. 38-44.(canceled)